included jitter correction on client

Client/udpstreamer/static/app.js

@@ -89,6 +89,15 @@ function getVScale(plotId, key) {
   return sigVScale[vsKey];
 }
 
+// Round a raw units-per-division up to the next 1/2/5×10ⁿ step so the Y-axis
+// gridlines land on human-readable values.
+function niceDiv(x) {
+  if (!isFinite(x) || x <= 0) return 1;
+  const p = Math.pow(10, Math.floor(Math.log10(x)));
+  const m = x / p;
+  return (m <= 1 ? 1 : m <= 2 ? 2 : m <= 5 ? 5 : 10) * p;
+}
+
 function findSignalMeta(key) {
   const colon = key.indexOf(':');
   if (colon < 0) return null;
@@ -107,8 +116,8 @@ function resolveVScale(plotId, key, rawY) {
   if (vs.mode === 'range') {
     const meta = findSignalMeta(key);
     if (meta && meta.rangeMin != null && meta.rangeMax != null && meta.rangeMax > meta.rangeMin) {
-      const divValue = (meta.rangeMax - meta.rangeMin) / 8;
+      const divValue = niceDiv((meta.rangeMax - meta.rangeMin) / 8);
-      const offset = (meta.rangeMin + meta.rangeMax) / 2;
+      const offset = Math.round((meta.rangeMin + meta.rangeMax) / 2 / divValue) * divValue;
       vs._resolvedDiv = divValue; vs._resolvedOffset = offset;
       return { divValue, offset, screenPos };
     }
@@ -120,7 +129,9 @@ function resolveVScale(plotId, key, rawY) {
     vs._resolvedDiv = divValue; vs._resolvedOffset = offset;
     return { divValue, offset, screenPos };
   }
-  // Auto: fit data in central 6 of 8 divisions, centered at screenPos
+  // Auto: fit data in central 6 of 8 divisions, centered at screenPos.
+  // Both the V/div and the centre offset are snapped so gridlines (and the
+  // zero line, when in view) fall on round values.
   let min = Infinity, max = -Infinity;
   for (let i = 0; i < rawY.length; i++) {
     const v = rawY[i];
@@ -128,8 +139,8 @@ function resolveVScale(plotId, key, rawY) {
   }
   if (!isFinite(min)) { min = -1; max = 1; }
   if (min === max) { min -= 1; max += 1; }
-  const divValue = Math.max((max - min) / 6, 1e-30);
+  const divValue = niceDiv(Math.max((max - min) / 6, 1e-30));
-  const offset = (max + min) / 2;
+  const offset = Math.round((max + min) / 2 / divValue) * divValue;
   vs._resolvedDiv = divValue; vs._resolvedOffset = offset;
   return { divValue, offset, screenPos };
 }
@@ -285,6 +296,12 @@ let _zoomFetchTimer = null;
 //   trig mode  → relative seconds from trigger
 const cursors = { mode: 'off', tA: null, tB: null };
 let cursorsDirty = false;  // if true, redraw all plots to update cursor lines
+// Rolling-window anchor used to keep cursors visually fixed while live data scrolls.
+let _cursorAnchorNow = null;
+
+// Horizontal value rulers — stored in normalized division units (the shared
+// y scale, -4.5…4.5) so one pair applies to every plot regardless of V/div.
+const rulers = { mode: 'off', yA: null, yB: null };
 
 // Layout — [label, cssClass, cols, rows]
 const LAYOUTS = [
@@ -329,7 +346,15 @@ async function resolveHub() {
 function connectWS() {
   ws = new WebSocket('ws://' + HUB + '/ws');
   ws.binaryType = 'arraybuffer';
-  ws.onopen = () => { wsBackoff = 1000; setStatus('orange', 'Connected – waiting for data'); };
+  ws.onopen = () => {
+    wsBackoff = 1000;
+    setStatus('orange', 'Connected – waiting for data');
+    // Restore monotonic TS preference from localStorage.
+    const monoPref = localStorage.getItem('udpscope.monotonic') === '1';
+    if (ws && ws.readyState === WebSocket.OPEN) {
+      ws.send(JSON.stringify({ type: 'setMonotonic', enabled: monoPref }));
+    }
+  };
   ws.onclose = () => {
     setStatus('red', 'Disconnected (reconnecting…)');
     setTimeout(connectWS, wsBackoff);
@@ -346,10 +371,26 @@ function connectWS() {
     else if (msg.type === 'zoom') onZoomReply(msg);
     else if (msg.type === 'historyZoom') onHistoryZoomReply(msg);
     else if (msg.type === 'historyInfo') onHistoryInfo(msg);
-    else if (msg.type === 'triggerState') onTriggerState(msg);
+    else if (msg.type === 'monotonicState') onMonotonicState(msg);
   };
 }
 
+/* Monotonic timestamp snapping — when enabled, the hub snaps small inter-frame
+   timestamp deviations (< 5 ms) to the ideal gap, eliminating overlaps/gaps
+   caused by software-dispatch jitter. */
+function onMonotonicState(msg) {
+  const cb = document.getElementById('cb-monotonic');
+  if (!cb) return;
+  cb.checked = !!msg.enabled;
+  localStorage.setItem('udpscope.monotonic', msg.enabled ? '1' : '0');
+}
+document.getElementById('cb-monotonic').addEventListener('change', e => {
+  localStorage.setItem('udpscope.monotonic', e.target.checked ? '1' : '0');
+  if (ws && ws.readyState === WebSocket.OPEN) {
+    ws.send(JSON.stringify({ type: 'setMonotonic', enabled: e.target.checked }));
+  }
+});
+
 /* WS zoom request/reply — replaces the Go hub's /api/zoom HTTP endpoint.
    Resolves with the {key:{t,v}} signals map; rejects on timeout/closure. */
 let _zoomReqId = 0;
@@ -882,24 +923,41 @@ function makeSeriesPath(key) {
 
 /* ════════════════════════════════════════════════════════════════
    LTTB Web Worker — offloads decimation off the main thread.
-   Cache key: "<plotId>:<masterKey>:<t0f>:<t1f>:<len>"
+   Cache key: "<plotId>:<masterKey>:<t0f>:<t1f>:<len>" when zoomed, or
+              "<plotId>:<masterKey>:rolling" in live mode, where the data
+              generation is passed alongside instead of being part of the key.
    On cache-hit  → render uses cached {t, v} immediately (stale-while-revalidate).
-   On cache-miss → render falls back to sync lttb once (first zoom render only),
+   On cache-miss → render falls back to sync lttb once (first render only),
                    then worker takes over for subsequent updates.
    ════════════════════════════════════════════════════════════════ */
-const lttbCache = new Map();   // key → {t, v}
+const lttbCache = new Map();   // key → {t, v, gen}
-const lttbPending = new Set(); // keys currently in-flight
+const lttbPending = new Map(); // key → generation currently in flight
+// Hard ceiling on cached decimations.  Rolling-mode keys are stable (one entry
+// per plot), but zoom keys embed the range, so without a cap the map would grow
+// for the whole session.
+const LTTB_CACHE_MAX = 256;
+
+// Store a decimation, re-inserting so Map iteration order stays oldest-first.
+function lttbCacheStore(key, entry) {
+  lttbCache.delete(key);
+  lttbCache.set(key, entry);
+  while (lttbCache.size > LTTB_CACHE_MAX) {
+    lttbCache.delete(lttbCache.keys().next().value);
+  }
+}
 
 let _lttbWorker = null;
 try {
   _lttbWorker = new Worker('lttb-worker.js');
   _lttbWorker.onmessage = function({ data: { id, t, v } }) {
+    const gen = lttbPending.get(id);
     lttbPending.delete(id);
-    lttbCache.set(id, { t, v });
+    lttbCacheStore(id, { t, v, gen });
-    // Invalidate and redraw the owning plot.
+    // Invalidate and redraw the owning plot.  Clearing lastDataGen defeats the
+    // render loop's no-new-data fast path so this fresh result is actually drawn.
     const plotId = parseInt(id.split(':')[0], 10);
     const p = plots.find(q => q.id === plotId);
-    if (p) { p.needsRedraw = true; }
+    if (p) { p.needsRedraw = true; p.lastDataGen = -1; }
   };
   _lttbWorker.onerror = e => console.warn('[lttb-worker] error:', e);
 } catch(e) {
@@ -907,14 +965,20 @@ try {
 }
 
 // Submit a LTTB job to the worker (or run sync if worker unavailable).
-// Returns cached {t, v} if fresh, null if a worker job was just submitted,
+// `gen` identifies the input data behind a key that does not itself change with
-// or a sync result if the worker is unavailable.
+// the data (rolling mode).  A cached entry computed for an older generation is
-function lttbAsync(cacheKey, t, v, threshold) {
+// still returned — stale-while-revalidate — while a fresh job runs.  Callers
+// whose key already encodes the input (zoom ranges) pass no generation.
+// Returns the cached {t, v} (fresh or stale), or null on the first render.
+function lttbAsync(cacheKey, t, v, threshold, gen) {
   const cached = lttbCache.get(cacheKey);
-  if (cached) return cached;                 // cache hit — use immediately
+  if (cached && cached.gen === gen) return cached;   // fresh — nothing to do
 
+  // At most one job per key in flight: submitting on every generation change
+  // would let the worker's message queue grow without bound whenever it cannot
+  // keep up with the push rate.
   if (!lttbPending.has(cacheKey)) {
-    lttbPending.add(cacheKey);
+    lttbPending.set(cacheKey, gen);
     if (_lttbWorker) {
       // Send copies so the main thread retains the originals.
       const tCopy = new Float64Array(t);
@@ -924,12 +988,13 @@ function lttbAsync(cacheKey, t, v, threshold) {
     } else {
       // Synchronous fallback (worker unavailable).
       const result = lttb(t, v, threshold);
-      lttbCache.set(cacheKey, result);
+      result.gen = gen;
+      lttbCacheStore(cacheKey, result);
       lttbPending.delete(cacheKey);
       return result;
     }
   }
-  return null;  // worker job in-flight — caller should use fallback
+  return cached || null;  // stale entry, or nothing to draw yet
 }
 
 // Evict stale LTTB cache entries for a plot (call when zoom range changes).
@@ -938,7 +1003,7 @@ function lttbCacheEvict(plotId) {
   for (const k of [...lttbCache.keys()]) {
     if (k.startsWith(prefix)) lttbCache.delete(k);
   }
-  for (const k of [...lttbPending]) {
+  for (const k of [...lttbPending.keys()]) {
     if (k.startsWith(prefix)) lttbPending.delete(k);
   }
 }
@@ -1447,6 +1512,51 @@ function drawCursorLines(u, p) {
   drawLine(cursors.tB, 'rgba(249,226,175,0.85)', 'B');
 }
 
+// Convert a normalized (division) y value to the active signal's raw units.
+function rulerRawValue(p, yNorm) {
+  const key = plotActiveSignal[p.id] || (p.traces.length === 1 ? p.traces[0] : null);
+  const vs = key ? sigVScale[p.id + ':' + key] : null;
+  if (!vs) return null;
+  const dv = vs._resolvedDiv != null ? vs._resolvedDiv : (vs.divValue || 1);
+  const ofs = vs._resolvedOffset != null ? vs._resolvedOffset : (vs.offset || 0);
+  return (yNorm - (vs.screenPos || 0)) * dv + ofs;
+}
+
+// Draw the horizontal value rulers (called from the draw hook).
+function drawRulerLines(u, p) {
+  if (rulers.mode !== 'on') return;
+  const { ctx, bbox } = u;
+  if (!bbox) return;
+
+  const drawLine = (yNorm, color, label) => {
+    if (yNorm === null) return;
+    const y = Math.round(u.valToPos(yNorm, 'y', true));
+    if (y < bbox.top || y > bbox.top + bbox.height) return;
+    ctx.save();
+    ctx.beginPath();
+    ctx.rect(bbox.left, bbox.top, bbox.width, bbox.height);
+    ctx.clip();
+    ctx.strokeStyle = color;
+    ctx.lineWidth = 1.5;
+    ctx.setLineDash([5, 4]);
+    ctx.beginPath();
+    ctx.moveTo(bbox.left, y);
+    ctx.lineTo(bbox.left + bbox.width, y);
+    ctx.stroke();
+    ctx.setLineDash([]);
+    ctx.fillStyle = color;
+    ctx.font = 'bold 11px monospace';
+    ctx.textAlign = 'left';
+    ctx.textBaseline = 'bottom';
+    const raw = p ? rulerRawValue(p, yNorm) : null;
+    ctx.fillText(label + (raw !== null ? ' ' + _fmtVal(raw) : ''), bbox.left + 4, y - 2);
+    ctx.restore();
+  };
+
+  drawLine(rulers.yA, 'rgba(166,227,161,0.85)', 'Y1');
+  drawLine(rulers.yB, 'rgba(243,139,168,0.85)', 'Y2');
+}
+
 // Compute the rolling-window anchor ("newest common timestamp") for a plot.
 // Returns the min-of-max timestamp across ACTIVE sources contributing traces to p,
 // so no live source shows a blank right edge.
@@ -1563,7 +1673,7 @@ function makeUPlotOpts(p, inTrigMode) {
     legend: { show: false },
     padding: [4, 4, 0, 0],
     hooks: {
-      draw: [u => { drawBandSeparators(u, p); drawActiveSeries(u, p); drawOffsetMarkers(u, p); drawCursorLines(u, p); drawSeriesMarkers(u, p); drawTriggerMarker(u, p); }],
+      draw: [u => { drawBandSeparators(u, p); drawActiveSeries(u, p); drawOffsetMarkers(u, p); drawCursorLines(u, p); drawRulerLines(u, p); drawSeriesMarkers(u, p); drawTriggerMarker(u, p); }],
       // Two-hook zoom detection: setSelect flags that the NEXT setScale is user-initiated.
       // uPlot fires setSelect → then immediately setScale (when drag.setScale:true).
       // All programmatic setScale calls happen without a preceding setSelect, so the
@@ -1617,34 +1727,60 @@ function createUPlot(p) {
     return min + pct * (max - min);
   }
 
-  // Update pointer style based on what's under the mouse
+  function _rulerAtClientY(clientY) {
+    const rect = p.uplot.over.getBoundingClientRect();
+    const { min, max } = p.uplot.scales.y;
+    const toY = val => rect.top + (1 - (val - min) / (max - min)) * rect.height;
+    if (rulers.yA !== null && Math.abs(clientY - toY(rulers.yA)) <= CURSOR_SNAP_PX) return 'A';
+    if (rulers.yB !== null && Math.abs(clientY - toY(rulers.yB)) <= CURSOR_SNAP_PX) return 'B';
+    return null;
+  }
+
+  function _rulerValFromEvent(e) {
+    const rect = p.uplot.over.getBoundingClientRect();
+    const pct = Math.max(0, Math.min(1, (e.clientY - rect.top) / rect.height));
+    const { min, max } = p.uplot.scales.y;
+    return max - pct * (max - min);
+  }
+
+  // Update pointer style based on what's under the mouse, and drive the
+  // time/value hover readout.
   p.uplot.over.addEventListener('mousemove', e => {
-    const snap = cursors.mode === 'on' ? _cursorAtClientX(e.clientX) : null;
+    const snapX = cursors.mode === 'on' ? _cursorAtClientX(e.clientX) : null;
-    p.uplot.over.style.cursor = snap ? 'ew-resize' : '';
+    const snapY = !snapX && rulers.mode === 'on' ? _rulerAtClientY(e.clientY) : null;
+    p.uplot.over.style.cursor = snapX ? 'ew-resize' : (snapY ? 'ns-resize' : '');
+    showHoverReadout(p, e);
   });
   p.uplot.over.addEventListener('mouseleave', () => {
     p.uplot.over.style.cursor = '';
+    hideHoverReadout();
   });
 
   // Mousedown: drag an existing cursor (only when mode='on' and mouse is near a cursor line).
   // If not near a cursor, the event falls through to uPlot for normal zoom/pan behavior.
   p.uplot.over.addEventListener('mousedown', e => {
     if (e.button !== 0 || e.shiftKey) return; // shift is pan
-    if (cursors.mode !== 'on') return;
+    const target = cursors.mode === 'on' ? _cursorAtClientX(e.clientX) : null;
-    const target = _cursorAtClientX(e.clientX);
+    const yTarget = !target && rulers.mode === 'on' ? _rulerAtClientY(e.clientY) : null;
-    if (!target) return; // not near a cursor — let uPlot handle zoom
+    if (!target && !yTarget) return; // not near a cursor — let uPlot handle zoom
 
     e.stopImmediatePropagation(); // prevent uPlot drag-zoom
     e.preventDefault();
 
     // Set cursor position immediately on mousedown
-    if (target === 'A') cursors.tA = _cursorValFromEvent(e);
+    if (yTarget) {
+      if (yTarget === 'A') rulers.yA = _rulerValFromEvent(e);
+      else rulers.yB = _rulerValFromEvent(e);
+    } else if (target === 'A') cursors.tA = _cursorValFromEvent(e);
     else cursors.tB = _cursorValFromEvent(e);
     updateCursorReadout();
     cursorsDirty = true;
 
     const onMove = ev => {
-      if (target === 'A') cursors.tA = _cursorValFromEvent(ev);
+      if (yTarget) {
+        if (yTarget === 'A') rulers.yA = _rulerValFromEvent(ev);
+        else rulers.yB = _rulerValFromEvent(ev);
+      } else if (target === 'A') cursors.tA = _cursorValFromEvent(ev);
       else cursors.tB = _cursorValFromEvent(ev);
       updateCursorReadout();
       cursorsDirty = true;
@@ -1871,10 +2007,12 @@ function buildLiveData(p) {
   // the full window slice can easily reach 100k–300k pts — far more than uPlot
   // needs for a 1200px-wide canvas.  Always run LTTB via the background worker
   // (stale-while-revalidate: use cached result; fall back to sync on first render).
-  // Rolling-mode cache key uses _dataGen so the result refreshes on new data.
+  // The rolling-mode key is constant per (plot, signal) — the data generation is
+  // carried separately so the cache holds one entry per plot instead of one per
+  // push tick, which used to grow without bound for the whole session.
   const targetPts = Math.max(LTTB_MIN, ((p.uplot ? p.uplot.width : p.div.clientWidth) || 600) * 2);
   const cacheKey = isRolling
-    ? `${p.id}:${masterKey}:rolling:${_dataGen}`
+    ? `${p.id}:${masterKey}:rolling`
     : `${p.id}:${masterKey}:${t0.toFixed(6)}:${t1.toFixed(6)}:${masterRaw.t.length}`;
   let sharedT, masterV;
   if (masterRaw.t.length <= targetPts) {
@@ -1882,7 +2020,8 @@ function buildLiveData(p) {
     sharedT = masterRaw.t;
     masterV = masterRaw.v;
   } else {
-    const cached = lttbAsync(cacheKey, masterRaw.t, masterRaw.v, targetPts);
+    const cached = lttbAsync(cacheKey, masterRaw.t, masterRaw.v, targetPts,
+                             isRolling ? _dataGen : undefined);
     let dec;
     if (cached) {
       dec = cached;
@@ -2094,19 +2233,10 @@ document.getElementById('btn-zoom-fit').addEventListener('click', zoomFit);
 /* ════════════════════════════════════════════════════════════════
    Cursor controls
    ════════════════════════════════════════════════════════════════ */
-// Show the cursor button only when paused or in trigger-snapshot mode.
+// Cursors are always available — in live rolling mode they are pinned to the
+// moving viewport by the render loop.
 function updateCursorBtnVisibility() {
-  const canUseCursors = globalPause || (trig.enabled && trig.snapshot !== null);
+  document.getElementById('btn-cursor').style.display = '';
-  const btn = document.getElementById('btn-cursor');
-  btn.style.display = canUseCursors ? '' : 'none';
-  if (!canUseCursors && cursors.mode !== 'off') {
-    cursors.mode = 'off';
-    cursors.tA = null; cursors.tB = null; // context changed, clear positions
-    btn.textContent = 'Cursors';
-    btn.classList.remove('active');
-    document.getElementById('cursor-readout').classList.remove('visible');
-    cursorsDirty = true;
-  }
 }
 
 document.getElementById('btn-cursor').addEventListener('click', () => {
@@ -2133,6 +2263,18 @@ document.getElementById('btn-cursor').addEventListener('click', () => {
   cursorsDirty = true;
 });
 
+document.getElementById('btn-ruler').addEventListener('click', () => {
+  rulers.mode = rulers.mode === 'off' ? 'on' : 'off';
+  const btn = document.getElementById('btn-ruler');
+  btn.classList.toggle('active', rulers.mode === 'on');
+  if (rulers.mode === 'on' && rulers.yA === null && rulers.yB === null) {
+    // Auto-place at ±2 divisions from the centre on first use.
+    rulers.yA = -2; rulers.yB = 2;
+  }
+  updateCursorReadout();
+  cursorsDirty = true;
+});
+
 // Format a signal value for the per-plot cursor readout.
 function fmtVal(v) {
   if (v === null || v === undefined) return '—';
@@ -2178,11 +2320,80 @@ function updatePlotCursorReadouts() {
   });
 }
 
+/* ─── Hover readout ──────────────────────────────────────────────────────── */
+// Un-normalize a plotted value of trace `key` in plot `p` back to raw units.
+function rawFromNorm(p, key, vNorm) {
+  const vs = sigVScale[p.id + ':' + key];
+  if (!vs) return vNorm;
+  const dv = vs._resolvedDiv != null ? vs._resolvedDiv : (vs.divValue || 1);
+  const ofs = vs._resolvedOffset != null ? vs._resolvedOffset : (vs.offset || 0);
+  return (vNorm - (vs.screenPos || 0)) * dv + ofs;
+}
+
+function hideHoverReadout() {
+  document.getElementById('hover-readout').style.display = 'none';
+}
+
+// Show the time under the mouse plus every trace's value at that time.
+function showHoverReadout(p, e) {
+  const el = document.getElementById('hover-readout');
+  if (!p.uplot || p.traces.length === 0) { el.style.display = 'none'; return; }
+
+  const rect = p.uplot.over.getBoundingClientRect();
+  const { min, max } = p.uplot.scales.x;
+  const pct = Math.max(0, Math.min(1, (e.clientX - rect.left) / rect.width));
+  const t = min + pct * (max - min);
+  const span = Math.abs(max - min);
+
+  const tStr = (trig.enabled && trig.snapshot) ? fmtDuration(t, span, true) : fmtLiveTime(t, span);
+  let html = '<div class="hov-time">' + escHtml(tStr) + '</div>';
+  p.traces.forEach((key, idx) => {
+    const vNorm = interpAtTime(p.uplot, idx + 1, t);
+    const name = key.includes(':') ? key.slice(key.indexOf(':') + 1) : key;
+    const val = vNorm === null ? '—' : _fmtVal(rawFromNorm(p, key, vNorm));
+    html += '<div class="hov-row"><span class="hov-dot" style="background:' +
+      escHtml(getSigStyle(key).color) + '"></span>' +
+      '<span class="hov-name">' + escHtml(name) + '</span>' +
+      '<span class="hov-val">' + escHtml(val) + '</span></div>';
+  });
+  el.innerHTML = html;
+  el.style.display = 'block';
+
+  // Keep the tooltip inside the viewport.
+  const w = el.offsetWidth, h = el.offsetHeight;
+  let x = e.clientX + 14, y = e.clientY + 14;
+  if (x + w > window.innerWidth - 4) x = e.clientX - w - 14;
+  if (y + h > window.innerHeight - 4) y = e.clientY - h - 14;
+  el.style.left = Math.max(4, x) + 'px';
+  el.style.top = Math.max(4, y) + 'px';
+}
+
+// Update the Y1/Y2/ΔY ruler readout, expressed in the raw units of the first
+// plot that has an active (or sole) signal.
+function updateRulerReadout() {
+  const box = document.getElementById('ruler-readout');
+  const on = rulers.mode === 'on';
+  box.style.display = on ? '' : 'none';
+  if (!on) return;
+  const ref = plots.find(p => p.uplot && p.traces.length > 0 &&
+                              rulerRawValue(p, 0) !== null);
+  const conv = y => (y === null || !ref) ? null : rulerRawValue(ref, y);
+  const vA = conv(rulers.yA), vB = conv(rulers.yB);
+  document.getElementById('cur-y1').textContent = 'Y1: ' + fmtVal(vA);
+  document.getElementById('cur-y2').textContent = 'Y2: ' + fmtVal(vB);
+  document.getElementById('cur-dy').textContent =
+    'ΔY: ' + fmtVal(vA !== null && vB !== null ? vB - vA : null);
+}
+
 function updateCursorReadout() {
   const ro = document.getElementById('cursor-readout');
   const active = cursors.mode === 'on';
-  ro.classList.toggle('visible', active);
+  ro.classList.toggle('visible', active || rulers.mode === 'on');
   updatePlotCursorReadouts();
+  updateRulerReadout();
+  ['cur-ta', 'cur-tb', 'cur-dt'].forEach(id => {
+    document.getElementById(id).style.display = active ? '' : 'none';
+  });
   if (!active) return;
 
   // Use the current visible x-range to pick the display unit.
@@ -2286,6 +2497,12 @@ document.getElementById('trig-mode').addEventListener('change', e => {
   }
 });
 document.getElementById('btn-trig-rearm').addEventListener('click', () => { if (trig.enabled) trigArm(); });
+// Force: capture the window around the newest sample regardless of the threshold.
+document.getElementById('btn-trig-force').addEventListener('click', () => {
+  if (!trig.enabled || !trig.signal) return;
+  sendTrigConfig();
+  wsSend({ type: 'forceTrigger' });
+});
 document.getElementById('btn-trig-stop').addEventListener('click', () => {
   if (!trig.enabled || trig.mode !== 'normal') return;
   trig.stopped = !trig.stopped;
@@ -2943,6 +3160,22 @@ function renderDirtyPlots() {
     });
   }
 
+  // Live rolling mode: pin the cursors to the moving viewport so they stay put
+  // on screen instead of scrolling off the left edge as time advances.
+  if (cursors.mode === 'on' && !trig.enabled && !globalPause &&
+      plots.some(p => p.uplot && !p.xRange && p.traces.length > 0)) {
+    if (_cursorAnchorNow !== null && globalPlotNow !== _cursorAnchorNow) {
+      const shift = globalPlotNow - _cursorAnchorNow;
+      if (cursors.tA !== null) cursors.tA += shift;
+      if (cursors.tB !== null) cursors.tB += shift;
+      cursorsDirty = true;
+      updateCursorReadout();
+    }
+    _cursorAnchorNow = globalPlotNow;
+  } else {
+    _cursorAnchorNow = null;
+  }
+
   // Fast path: cursor-only redraw (no data rebuild needed)
   if (cursorsDirty) {
     cursorsDirty = false;
@@ -3204,11 +3437,15 @@ function showVScaleMenu(key, plotId) {
 
   const isManual = vs.mode === 'manual';
   document.getElementById('vscale-manual-row').style.display = isManual ? 'flex' : 'none';
+  document.getElementById('vscale-offset-row').style.display = isManual ? 'flex' : 'none';
   document.getElementById('vscale-pos-row').style.display = isManual ? 'flex' : 'none';
 
-  // Pre-fill V/div with resolved or stored value; Position always shows current screenPos.
+  // Pre-fill V/div and Offset with the resolved or stored values; Position
+  // always shows the current screenPos.
   const dv = isManual ? vs.divValue : (vs._resolvedDiv || 1);
+  const ofs = isManual ? (vs.offset || 0) : (vs._resolvedOffset || 0);
   document.getElementById('vscale-vdiv').value = dv != null ? parseFloat(dv.toPrecision(4)) : 1;
+  document.getElementById('vscale-offset').value = parseFloat(ofs.toPrecision(6));
   document.getElementById('vscale-pos').value = parseFloat((vs.screenPos || 0).toPrecision(4));
 
   // Type row (Analog/Digital) only shown in mixed mode.
@@ -3293,8 +3530,9 @@ function initVScaleMenu() {
       if (newMode === 'manual' && vs.mode !== 'manual') {
         // Seed V/div from currently resolved value; screenPos stays as-is.
         vs.divValue = vs._resolvedDiv || 1;
-        vs.offset = vs._resolvedOffset || 0; // keep for DC subtraction (internal)
+        vs.offset = vs._resolvedOffset || 0; // raw value at screen centre
         document.getElementById('vscale-vdiv').value = parseFloat(vs.divValue.toPrecision(4));
+        document.getElementById('vscale-offset').value = parseFloat(vs.offset.toPrecision(6));
         document.getElementById('vscale-pos').value = parseFloat((vs.screenPos || 0).toPrecision(4));
       }
       vs.mode = newMode;
@@ -3302,6 +3540,7 @@ function initVScaleMenu() {
       btn.classList.add('active');
       const isManual = vs.mode === 'manual';
       document.getElementById('vscale-manual-row').style.display = isManual ? 'flex' : 'none';
+      document.getElementById('vscale-offset-row').style.display = isManual ? 'flex' : 'none';
       document.getElementById('vscale-pos-row').style.display = isManual ? 'flex' : 'none';
       refreshPlotForKey(_vsMenuKey);
     });
@@ -3312,6 +3551,16 @@ function initVScaleMenu() {
     vs.divValue = Math.max(parseFloat(e.target.value) || 1, 1e-30);
     refreshPlotForKey(_vsMenuKey);
   });
+  // "Offset" is the raw value shown at screen centre.  Unlike Position (which
+  // is clamped to the ±4 visible divisions) it is unbounded, so a signal can be
+  // referenced to a level far outside the currently plotted range.
+  document.getElementById('vscale-offset').addEventListener('input', e => {
+    if (!_vsMenuKey) return;
+    const vs = getVScale(_vsMenuPlotId, _vsMenuKey);
+    const v = parseFloat(e.target.value);
+    vs.offset = isFinite(v) ? v : 0;
+    refreshPlotForKey(_vsMenuKey);
+  });
   // "Position (div)" moves the marker and signal together on screen.
   document.getElementById('vscale-pos').addEventListener('input', e => {
     if (!_vsMenuKey) return;
@@ -3551,6 +3800,11 @@ buildSidebar();   // show "Add Source" section even before WS connection
 initArrayIdxPicker();
 initVScaleMenu();
 initSignalMenu();
+// Restore monotonic TS checkbox from localStorage (visual state before WS reply).
+{
+  const cb = document.getElementById('cb-monotonic');
+  if (cb) cb.checked = localStorage.getItem('udpscope.monotonic') === '1';
+}
 document.getElementById('btn-csv-all').addEventListener('click', exportAllCSV);
 document.getElementById('btn-stats').addEventListener('click', toggleStats);
 document.getElementById('btn-stats-close').addEventListener('click', toggleStats);

Client/udpstreamer/static/index.html

@@ -21,6 +21,12 @@
         <span id="cur-ta">A: —</span><span class="cur-sep">│</span>
         <span id="cur-tb">B: —</span><span class="cur-sep">│</span>
         <span id="cur-dt">ΔT: —</span>
+        <span id="ruler-readout" style="display:none">
+          <span class="cur-sep">│</span>
+          <span id="cur-y1">Y1: —</span><span class="cur-sep">│</span>
+          <span id="cur-y2">Y2: —</span><span class="cur-sep">│</span>
+          <span id="cur-dy">ΔY: —</span>
+        </span>
       </div>
       <span class="ctrl-label" id="lbl-window">Window:</span>
       <select id="window-select" class="ctrl-select">
@@ -28,13 +34,17 @@
         <option value="10">10 s</option><option value="30">30 s</option>
         <option value="60">60 s</option>
       </select>
-      <button id="btn-cursor" class="ctrl-btn" style="display:none">Cursor</button>
+      <button id="btn-cursor" class="ctrl-btn">Cursors</button>
+      <button id="btn-ruler" class="ctrl-btn" title="Horizontal value rulers">Rulers</button>
       <button id="btn-zoom-back" class="ctrl-btn" style="display:none">← Back</button>
       <button id="btn-zoom-fit" class="ctrl-btn">Fit</button>
       <button id="btn-csv-all" class="ctrl-btn" title="Export all signals to CSV">⬇ CSV</button>
       <button id="btn-sync-resume" class="ctrl-btn resume-btn" style="display:none">↺ Auto</button>
       <button id="btn-trigger" class="ctrl-btn">⚡ Trigger</button>
       <button id="btn-pause-global" class="ctrl-btn">⏸ Pause</button>
+      <label class="ctrl-check" title="Snap jittery inter-frame timestamps to ideal spacing (eliminates overlaps/gaps from software-dispatch jitter)">
+        <input type="checkbox" id="cb-monotonic"> Sync TS
+      </label>
     </div>
     <!-- ── Trigger bar ───────────────────────────────────────────── -->
     <div id="trigbar">
@@ -83,6 +93,7 @@
       <div class="trig-sep"></div>
       <div class="trig-group" style="gap:8px">
         <span id="trig-status-badge">IDLE</span>
+        <button id="btn-trig-force" title="Capture now, ignoring the threshold">Force</button>
         <button id="btn-trig-stop" style="display:none">Stop</button>
         <button id="btn-trig-rearm">Rearm</button>
       </div>
@@ -185,6 +196,10 @@
           <label class="vstb-lbl">V/div</label>
           <input type="number" id="vscale-vdiv" class="ctx-num" min="1e-30" step="any" value="1">
         </div>
+        <div id="vscale-offset-row" style="display:none;align-items:center;gap:4px">
+          <label class="vstb-lbl" title="Raw value at screen centre — unbounded, may lie outside the plotted range">Offset</label>
+          <input type="number" id="vscale-offset" class="ctx-num" step="any" value="0">
+        </div>
         <div id="vscale-pos-row" style="display:none;align-items:center;gap:4px">
           <label class="vstb-lbl">Pos</label>
           <input type="number" id="vscale-pos" class="ctx-num" step="0.1" value="0">
@@ -199,6 +214,8 @@
         <button id="btn-vscale-close" class="vstb-close" title="Close">✕</button>
       </div>
     </div>
+    <!-- Follows the mouse over a plot: time + per-trace values. -->
+    <div id="hover-readout" style="display:none"></div>
     <script src="/app.js"></script>
   </body>
 </html>

Client/udpstreamer/static/style.css

@@ -52,6 +52,23 @@ html, body { height:100%; background:var(--bg); color:var(--text);
 #cursor-readout.visible { display:flex; }
 #cur-ta { color:var(--sky); } #cur-tb { color:var(--yellow); }
 #cur-dt { color:var(--subtext1); } .cur-sep { color:var(--surface2); }
+#ruler-readout { display:inline-flex; align-items:center; gap:8px; }
+#cur-y1 { color:var(--green); } #cur-y2 { color:var(--red); }
+#cur-dy { color:var(--subtext1); }
+
+/* Mouse-over time/value tooltip */
+#hover-readout {
+  position:fixed; z-index:60; pointer-events:none;
+  background:var(--surface0); border:1px solid var(--surface1);
+  border-radius:5px; padding:4px 8px;
+  font-size:11px; font-family:monospace; white-space:nowrap;
+  box-shadow:0 4px 12px rgba(0,0,0,0.45);
+}
+#hover-readout .hov-time { color:var(--subtext1); margin-bottom:3px; }
+#hover-readout .hov-row  { display:flex; align-items:center; gap:6px; }
+#hover-readout .hov-dot  { width:8px; height:8px; border-radius:50%; flex-shrink:0; }
+#hover-readout .hov-name { color:var(--subtext0); }
+#hover-readout .hov-val  { color:var(--text); margin-left:auto; padding-left:10px; }
 
 .topbar-vsep { width:1px; height:22px; background:var(--surface0); flex-shrink:0; margin:0 2px; }
 #layout-btns { display:flex; gap:2px; align-items:center; flex-shrink:0; }
@@ -74,6 +91,12 @@ button.ctrl-btn.trig-active { background:rgba(203,166,247,0.15); border-color:va
 button.ctrl-btn.cursor-a    { border-color:var(--sky);   color:var(--sky); }
 button.ctrl-btn.cursor-b    { border-color:var(--yellow); color:var(--yellow); }
 button.ctrl-btn.resume-btn  { border-color:var(--teal);  color:var(--teal); }
+label.ctrl-check {
+  display:flex; align-items:center; gap:4px; flex-shrink:0;
+  font-size:12px; color:var(--subtext0); cursor:pointer; white-space:nowrap;
+}
+label.ctrl-check input { margin:0; cursor:pointer; accent-color:var(--accent); }
+label.ctrl-check:has(input:checked) { color:var(--accent); }
 
 /* ── Trigger bar ──────────────────────────────────────────────── */
 #trigbar {

Common/Client/go/wshub/hub.go

@@ -107,7 +107,18 @@ func (c *wsClient) readPump() {
 					case c.hub.commandCh <- hubCmd{op: "wsSaveSources"}:
 					default:
 					}
+				case "setMonotonic":
+					enabled, _ := env["enabled"].(bool)
+					select {
+					case c.hub.commandCh <- hubCmd{op: "setMonotonic", enabled: enabled}:
+					default:
+					}
+				case "zoom":
+					c.hub.handleWSZoom(c, env)
 				default:
+					if c.hub.handleTriggerCommand(t, env) {
+						break
+					}
 					// Unrecognized message type — forward to DebugCh
 					select {
 					case c.hub.DebugCh <- msg:
@@ -182,6 +193,14 @@ type sourceHubState struct {
 	// per signal name.  Used by the default (TimeModePacket, n>1) path to estimate
 	// per-element dt when only one packet arrives in a 30 Hz tick.
 	lastPktNs map[string]int64
+
+	// Monotonic timestamp snapping state (all accessed from Run() goroutine):
+	// lastFrameMeasured — uncorrected measured anchor of the previous frame.
+	// lastFrameEndT     — corrected anchor after snapping.
+	// gapEMA            — exponential moving average of the measured inter-frame gap.
+	lastFrameMeasured map[string]float64
+	lastFrameEndT     map[string]float64
+	gapEMA            map[string]float64
 }
 
 // taggedSample is a DataSample annotated with its source ID.
@@ -192,7 +211,7 @@ type taggedSample struct {
 
 // hubCmd carries a command to the Run() goroutine.
 type hubCmd struct {
-	op       string // "addSource","removeSource","setSourceState","updateConfig",
+	op string // "addSource","removeSource","setSourceState","updateConfig",
 	//                "wsAddSource","wsRemoveSource","wsSaveSources"
 	sourceID       string
 	label          string
@@ -201,6 +220,7 @@ type hubCmd struct {
 	sigs           []udpsprotocol.SignalInfo
 	multicastGroup string
 	dataPort       int
+	enabled        bool // "setMonotonic" toggle
 }
 
 // Hub is the central broker between UDP clients and WebSocket clients.
@@ -224,21 +244,17 @@ type Hub struct {
 	ringsMu sync.RWMutex
 	rings   map[string]*sigRing // "sourceId:signalKey" → ring
 
-	// lastZoomAt tracks the last time a zoom request was served.
-	// Ring buffer writes are skipped when no zoom has been requested
-	// in the last 10 s, saving substantial CPU on LTTB + ring writes.
-	lastZoomAt time.Time
-	zoomAtMu   sync.Mutex
-
 	statsMu  sync.RWMutex
 	statsMap map[string]*SourceStat
 
-	// onClientConnect, if set, is called each time a new WebSocket client
+	// trigger is the hub-side trigger FSM driving the oscilloscope capture mode.
-	// registers.  The callback receives a send function that delivers a message
+	trigger           *triggerEngine
-	// directly to that client.  It is invoked synchronously from Run(), so it
-	// must not block.
 	onClientConnectMu sync.RWMutex
 	onClientConnect   func(send func([]byte))
+
+	// monotonicTS, when true, snaps small inter-frame timestamp deviations
+	// (< monotonicTolerance) to the ideal gap to eliminate jitter.
+	monotonicTS bool
 }
 
 // NewHub creates an initialised Hub.
@@ -253,6 +269,7 @@ func NewHub() *Hub {
 		DebugCh:     make(chan []byte, 256),
 		rings:       make(map[string]*sigRing),
 		statsMap:    make(map[string]*SourceStat),
+		trigger:     newTriggerEngine(),
 	}
 }
 
@@ -278,44 +295,9 @@ func (h *Hub) getRing(key string) *sigRing {
 	return rb
 }
 
-// shouldWriteRing returns true if zoom was requested within the last 10 seconds.
+// zoomSlice extracts [t0, t1] from the full-resolution rings for the named
-func (h *Hub) shouldWriteRing() bool {
+// signals, decimating each to at most n points.
-	h.zoomAtMu.Lock()
+func (h *Hub) zoomSlice(t0, t1 float64, keys []string, n int) map[string]sigData {
-	ok := time.Since(h.lastZoomAt) < 10*time.Second
-	h.zoomAtMu.Unlock()
-	return ok
-}
-
-// HandleZoom serves GET /api/zoom?... It also records the access time
-// so the ring buffer knows zoom is active and worth populating.
-func (h *Hub) HandleZoom(w http.ResponseWriter, r *http.Request) {
-	q := r.URL.Query()
-	t0, err0 := strconv.ParseFloat(q.Get("t0"), 64)
-	t1, err1 := strconv.ParseFloat(q.Get("t1"), 64)
-	if err0 != nil || err1 != nil || t1 <= t0 {
-		http.Error(w, "invalid t0/t1", http.StatusBadRequest)
-		return
-	}
-	var n int
-	if nStr := q.Get("n"); nStr == "" {
-		n = 2400
-	} else {
-		n, _ = strconv.Atoi(nStr)
-		if n <= 0 {
-			n = 1 << 30 // no decimation
-		} else if n < 10 {
-			n = 2400
-		}
-	}
-
-	if n > 0 {
-		h.zoomAtMu.Lock()
-		h.lastZoomAt = time.Now()
-		h.zoomAtMu.Unlock()
-	}
-
-	keys := strings.Split(q.Get("signals"), ",")
-
 	h.ringsMu.RLock()
 	refs := make(map[string]*sigRing, len(keys))
 	for _, k := range keys {
@@ -338,11 +320,69 @@ func (h *Hub) HandleZoom(w http.ResponseWriter, r *http.Request) {
 		dt, dv := lttbDecimate(rt, rv, n)
 		result[k] = sigData{T: dt, V: dv}
 	}
+	return result
+}
+
+// zoomPoints normalises the client's requested point budget: absent → 2400,
+// non-positive → every sample in the range, implausibly small → 2400.
+func zoomPoints(n int, present bool) int {
+	switch {
+	case !present:
+		return 2400
+	case n <= 0:
+		return 1 << 30 // no decimation
+	case n < 10:
+		return 2400
+	}
+	return n
+}
+
+// handleWSZoom answers a browser {"type":"zoom","reqId":..,"t0":..,"t1":..,
+// "n":..,"signals":"a,b"} request, unicasting {"type":"zoom","reqId":..,
+// "signals":{...}} back to the requesting client.  This is the path the web SPA
+// actually uses; /api/zoom is the equivalent HTTP entry point.
+func (h *Hub) handleWSZoom(c *wsClient, env map[string]interface{}) {
+	t0, ok0 := env["t0"].(float64)
+	t1, ok1 := env["t1"].(float64)
+	if !ok0 || !ok1 || t1 <= t0 {
+		return
+	}
+	nF, nOK := env["n"].(float64)
+	n := zoomPoints(int(nF), nOK)
+	sigCSV, _ := env["signals"].(string)
+
+	reply, err := json.Marshal(map[string]any{
+		"type":    "zoom",
+		"reqId":   env["reqId"],
+		"signals": h.zoomSlice(t0, t1, strings.Split(sigCSV, ","), n),
+	})
+	if err != nil {
+		log.Printf("hub: ws zoom encode: %v", err)
+		return
+	}
+	select {
+	case c.send <- wsMessage{websocket.TextMessage, reply}:
+	default:
+	}
+}
+
+// HandleZoom serves GET /api/zoom?...
+func (h *Hub) HandleZoom(w http.ResponseWriter, r *http.Request) {
+	q := r.URL.Query()
+	t0, err0 := strconv.ParseFloat(q.Get("t0"), 64)
+	t1, err1 := strconv.ParseFloat(q.Get("t1"), 64)
+	if err0 != nil || err1 != nil || t1 <= t0 {
+		http.Error(w, "invalid t0/t1", http.StatusBadRequest)
+		return
+	}
+	nStr := q.Get("n")
+	nVal, _ := strconv.Atoi(nStr)
+	n := zoomPoints(nVal, nStr != "")
 
 	w.Header().Set("Content-Type", "application/json")
 	if err := json.NewEncoder(w).Encode(map[string]any{
 		"type":    "zoom",
-		"signals": result,
+		"signals": h.zoomSlice(t0, t1, strings.Split(q.Get("signals"), ","), n),
 	}); err != nil {
 		log.Printf("hub: zoom encode: %v", err)
 	}
@@ -455,13 +495,28 @@ func (h *Hub) Run() {
 			h.clients[c] = true
 			// Send current state to the new client.
 			if sourcesMsg != nil {
-				select { case c.send <- wsMessage{websocket.TextMessage, sourcesMsg}: default: }
+				select {
+				case c.send <- wsMessage{websocket.TextMessage, sourcesMsg}:
+				default:
+				}
 			}
 			for _, src := range sourcesMap {
 				if src.configJS != nil {
-					select { case c.send <- wsMessage{websocket.TextMessage, src.configJS}: default: }
+					select {
+					case c.send <- wsMessage{websocket.TextMessage, src.configJS}:
+					default:
+					}
 				}
 			}
+			select {
+			case c.send <- wsMessage{websocket.TextMessage, h.trigger.stateMsg()}:
+			default:
+			}
+			monoMsg, _ := json.Marshal(map[string]any{"type": "monotonicState", "enabled": h.monotonicTS})
+			select {
+			case c.send <- wsMessage{websocket.TextMessage, monoMsg}:
+			default:
+			}
 			// Notify the application layer so it can replay any persistent state
 			// (e.g., MARTe2 connection status, forced/traced signals).
 			h.onClientConnectMu.RLock()
@@ -469,7 +524,10 @@ func (h *Hub) Run() {
 			h.onClientConnectMu.RUnlock()
 			if fn != nil {
 				fn(func(msg []byte) {
-					select { case c.send <- wsMessage{websocket.TextMessage, msg}: default: }
+					select {
+					case c.send <- wsMessage{websocket.TextMessage, msg}:
+					default:
+					}
 				})
 			}
 
@@ -481,19 +539,25 @@ func (h *Hub) Run() {
 
 		case msg := <-h.broadcastCh:
 			for c := range h.clients {
-				select { case c.send <- wsMessage{websocket.TextMessage, msg}: default: }
+				select {
+				case c.send <- wsMessage{websocket.TextMessage, msg}:
+				default:
+				}
 			}
 
 		case cmd := <-h.commandCh:
 			switch cmd.op {
 			case "addSource":
 				sourcesMap[cmd.sourceID] = &sourceHubState{
-					id:           cmd.sourceID,
+					id:                cmd.sourceID,
-					label:        cmd.label,
+					label:             cmd.label,
-					addr:         cmd.addr,
+					addr:              cmd.addr,
-					connState:    "connecting",
+					connState:         "connecting",
-					timeSigCalib: make(map[string]float64),
+					timeSigCalib:      make(map[string]float64),
-					lastPktNs:    make(map[string]int64),
+					lastPktNs:         make(map[string]int64),
+					lastFrameEndT:     make(map[string]float64),
+					lastFrameMeasured: make(map[string]float64),
+					gapEMA:            make(map[string]float64),
 				}
 				h.statsMu.Lock()
 				h.statsMap[cmd.sourceID] = &SourceStat{}
@@ -529,6 +593,7 @@ func (h *Hub) Run() {
 				}
 				src.signals = cmd.sigs
 				src.configSeq++
+				src.lastFrameEndT = make(map[string]float64)
 				cfgMsg, err := json.Marshal(map[string]any{
 					"type":     "config",
 					"sourceId": cmd.sourceID,
@@ -581,6 +646,10 @@ func (h *Hub) Run() {
 						log.Printf("hub: save sources: %v", err)
 					}
 				}
+			case "setMonotonic":
+				h.monotonicTS = cmd.enabled
+				monoMsg, _ := json.Marshal(map[string]any{"type": "monotonicState", "enabled": h.monotonicTS})
+				h.broadcast(monoMsg)
 			}
 
 		case ts := <-h.dataCh:
@@ -607,6 +676,7 @@ func (h *Hub) Run() {
 					}
 				}
 			}
+			h.triggerTick()
 
 		case <-statsTicker.C:
 			h.statsMu.RLock()
@@ -640,11 +710,26 @@ func writeFloat64s(buf []byte, off int, f []float64) int {
 
 // ─── Data serialisation ───────────────────────────────────────────────────────
 
+// maxPushPoints bounds the live push only.  The zoom rings deliberately store
+// every sample: decimating on the way in would cap the resolution a zoom can
+// ever recover, and the browser already decimates for display.
 const maxPushPoints = 50
-const maxRingPoints = 20_000
+
+// Zoom ring depth, in samples per signal (16 bytes each).  ringCapTemporal
+// holds 6 s of a 1 MSps waveform; ringCapScalar holds 100 000 packets.
 const ringCapTemporal = 6_000_000
 const ringCapScalar = 100_000
 
+// monotonicTolerance is the maximum inter-frame timestamp deviation (seconds)
+// treated as jitter and snapped to the ideal gap.  Larger deviations are
+// preserved as genuine discontinuities (missing frames, rate changes).
+const monotonicTolerance = 0.005 // 5 ms
+
+// monotonicEMAAlpha is the smoothing factor for the inter-frame gap EMA.
+// 0.01 gives a time constant of ~100 frames (~1 s at 100 Hz): fast enough to
+// track real rate changes, slow enough to average out per-frame jitter.
+const monotonicEMAAlpha = 0.01
+
 // lttbDecimate reduces (tIn, vIn) to at most threshold representative points
 // using the Largest-Triangle-Three-Buckets algorithm.
 func lttbDecimate(tIn, vIn []float64, threshold int) ([]float64, []float64) {
@@ -667,10 +752,13 @@ func lttbDecimate(tIn, vIn []float64, threshold int) ([]float64, []float64) {
 		}
 		avgT, avgV, cnt := 0.0, 0.0, 0
 		for j := avgS; j < avgE; j++ {
-			avgT += tIn[j]; avgV += vIn[j]; cnt++
+			avgT += tIn[j]
+			avgV += vIn[j]
+			cnt++
 		}
 		if cnt > 0 {
-			avgT /= float64(cnt); avgV /= float64(cnt)
+			avgT /= float64(cnt)
+			avgV /= float64(cnt)
 		}
 		rS := int(float64(i)*every) + 1
 		rE := int(float64(i+1)*every) + 1
@@ -682,7 +770,8 @@ func lttbDecimate(tIn, vIn []float64, threshold int) ([]float64, []float64) {
 		for j := rS; j < rE; j++ {
 			area := math.Abs((aT-avgT)*(vIn[j]-aV) - (aT-tIn[j])*(avgV-aV))
 			if area > maxArea {
-				maxArea = area; next = j
+				maxArea = area
+				next = j
 			}
 		}
 		outT[i+1], outV[i+1] = tIn[next], vIn[next]
@@ -710,11 +799,13 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 	if src.configSeq != src.configSeqAtCalib {
 		src.configSeqAtCalib = src.configSeq
 		src.timeSigCalib = make(map[string]float64)
+		src.lastFrameEndT = make(map[string]float64)
+		src.lastFrameMeasured = make(map[string]float64)
+		src.gapEMA = make(map[string]float64)
 	}
 
 	sigs := src.signals
 	pfx := src.id + ":"
-	writeRing := h.shouldWriteRing()
 
 	type pairBuf struct {
 		t, v []float64
@@ -766,6 +857,25 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 					anchorTime = float64(s.WallTime.UnixNano()) / 1e9
 					anchorIsFirstSample = false
 				}
+				if h.monotonicTS && dt > 0 {
+					nominalGap := float64(n) * dt
+					measuredAnchor := anchorTime
+					if prevMeasured, ok := src.lastFrameMeasured[sig.Name]; ok {
+						measuredGap := measuredAnchor - prevMeasured
+						prevEMA, hasEMA := src.gapEMA[sig.Name]
+						if !hasEMA {
+							prevEMA = nominalGap
+						}
+						src.gapEMA[sig.Name] = prevEMA*(1-monotonicEMAAlpha) + measuredGap*monotonicEMAAlpha
+						smoothedGap := src.gapEMA[sig.Name]
+						deviation := math.Abs(measuredGap - smoothedGap)
+						if deviation > 0 && deviation < monotonicTolerance {
+							anchorTime = src.lastFrameEndT[sig.Name] + smoothedGap
+						}
+					}
+					src.lastFrameMeasured[sig.Name] = measuredAnchor
+					src.lastFrameEndT[sig.Name] = anchorTime
+				}
 				for k := 0; k < n; k++ {
 					var t float64
 					if anchorIsFirstSample {
@@ -777,12 +887,10 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 					allV = append(allV, vals[k])
 				}
 			}
-			if writeRing {
+			if rb := h.getRing(pfx + sig.Name); rb != nil {
-				ringT, ringV := lttbDecimate(allT, allV, maxRingPoints)
+				rb.write(allT, allV)
-				if rb := h.getRing(pfx + sig.Name); rb != nil {
-					rb.write(ringT, ringV)
-				}
 			}
+			h.trigger.feed(pfx+sig.Name, n, allT, allV)
 			decimT, decimV := lttbDecimate(allT, allV, maxPushPoints)
 			pairs[sig.Name] = pairBuf{t: decimT, v: decimV}
 
@@ -825,12 +933,10 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 					allV = append(allV, vals[k])
 				}
 			}
-			if writeRing {
+			if rb := h.getRing(pfx + sig.Name); rb != nil {
-				ringT, ringV := lttbDecimate(allT, allV, maxRingPoints)
+				rb.write(allT, allV)
-				if rb := h.getRing(pfx + sig.Name); rb != nil {
-					rb.write(ringT, ringV)
-				}
 			}
+			h.trigger.feed(pfx+sig.Name, n, allT, allV)
 			decimT, decimV := lttbDecimate(allT, allV, maxPushPoints)
 			pairs[sig.Name] = pairBuf{t: decimT, v: decimV}
 
@@ -845,25 +951,22 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 				ts = append(ts, float64(s.WallTime.UnixNano())/1e9)
 				vs = append(vs, vals[0])
 			}
-			if writeRing {
+			if rb := h.getRing(pfx + sig.Name); rb != nil {
-				if rb := h.getRing(pfx + sig.Name); rb != nil {
+				rb.write(ts, vs)
-					rb.write(ts, vs)
-				}
 			}
+			h.trigger.feed(pfx+sig.Name, 1, ts, vs)
 			pairs[sig.Name] = pairBuf{t: ts, v: vs}
 
 		default:
 			// n > 1, TimeModePacket: C++ sends samplingRate=0 so we interpolate
 			// per-element timestamps from wall-clock differences between packets.
 			//
-			// Three fixes vs the naïve approach:
+			// Two fixes vs the naïve approach:
 			//   1. Use src.lastPktNs[name] for the single-packet case so dt is
 			//      estimated from the actual inter-packet gap, not 1/n.
 			//   2. Send all n elements to the browser without LTTB so sinusoidal
 			//      waveforms are not degraded (packets arrive at ≤30 Hz, bandwidth
 			//      is trivially acceptable).
-			//   3. Always write the ring buffer regardless of shouldWriteRing() so
-			//      the first zoom request immediately returns full-resolution data.
 			allT := make([]float64, 0, len(batch)*n)
 			allV := make([]float64, 0, len(batch)*n)
 			for bi, s := range batch {
@@ -876,19 +979,38 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 				var dtSec float64
 				if bi+1 < len(batch) {
 					// Two consecutive packets in this tick → exact dt.
-					dtSec = (float64(batch[bi+1].WallTime.UnixNano())-float64(wallNs))/1e9/float64(n)
+					dtSec = (float64(batch[bi+1].WallTime.UnixNano()) - float64(wallNs)) / 1e9 / float64(n)
 				} else if bi > 0 {
 					// Last of multiple packets → use diff from previous.
-					dtSec = (float64(wallNs)-float64(batch[bi-1].WallTime.UnixNano()))/1e9/float64(n)
+					dtSec = (float64(wallNs) - float64(batch[bi-1].WallTime.UnixNano())) / 1e9 / float64(n)
 				} else if prevNs, ok2 := src.lastPktNs[sig.Name]; ok2 && prevNs > 0 && wallNs > prevNs {
 					// Single packet this tick → gap from the previous tick's packet.
-					dtSec = (float64(wallNs)-float64(prevNs))/1e9/float64(n)
+					dtSec = (float64(wallNs) - float64(prevNs)) / 1e9 / float64(n)
 				} else {
 					// Truly first packet ever — inter-packet timing unknown.
 					// Skip to avoid poisoning the ring with wrongly-spaced timestamps;
 					// lastPktNs will be recorded below so the next packet uses correct dt.
 					continue
 				}
+				if h.monotonicTS && dtSec > 0 {
+					nominalGap := float64(n) * dtSec
+					measuredStart := wallSec
+					if prevMeasured, ok := src.lastFrameMeasured[sig.Name]; ok {
+						measuredGap := measuredStart - prevMeasured
+						prevEMA, hasEMA := src.gapEMA[sig.Name]
+						if !hasEMA {
+							prevEMA = nominalGap
+						}
+						src.gapEMA[sig.Name] = prevEMA*(1-monotonicEMAAlpha) + measuredGap*monotonicEMAAlpha
+						smoothedGap := src.gapEMA[sig.Name]
+						deviation := math.Abs(measuredGap - smoothedGap)
+						if deviation > 0 && deviation < monotonicTolerance {
+							wallSec = src.lastFrameEndT[sig.Name] + smoothedGap
+						}
+					}
+					src.lastFrameMeasured[sig.Name] = measuredStart
+					src.lastFrameEndT[sig.Name] = wallSec
+				}
 				for j := 0; j < n; j++ {
 					allT = append(allT, wallSec+float64(j)*dtSec)
 					allV = append(allV, vals[j])
@@ -898,11 +1020,10 @@ func (h *Hub) buildBinaryDataMessageForSource(src *sourceHubState, batch []udpsp
 				src.lastPktNs[sig.Name] = batch[len(batch)-1].WallTime.UnixNano()
 			}
 			if len(allT) > 0 {
-				// Ring: always populate (fix 3), LTTB only if it actually reduces size.
-				ringT, ringV := lttbDecimate(allT, allV, maxRingPoints)
 				if rb := h.getRing(pfx + sig.Name); rb != nil {
-					rb.write(ringT, ringV)
+					rb.write(allT, allV)
 				}
+				h.trigger.feed(pfx+sig.Name, n, allT, allV)
 				// Live push: send all points without LTTB (fix 2).
 				pairs[sig.Name] = pairBuf{t: allT, v: allV}
 			}

Source/Applications/StreamHub/StreamHub.cpp

@@ -750,6 +750,7 @@ void StreamHub::OnWSCommand(const char *json, uint32 /*len*/, uint32 slotIdx) {
     else if (strcmp(type, "rearm")        == 0) { HandleRearm();            }
     else if (strcmp(type, "trigStop")     == 0) { HandleTrigStop(json);     }
     else if (strcmp(type, "setTrigger")   == 0) { HandleSetTrigger(json);   }
+    else if (strcmp(type, "forceTrigger") == 0) { HandleForceTrigger();     }
     else if (strcmp(type, "zoom")         == 0) { HandleZoom(json, slotIdx); }
     else if (strcmp(type, "historyZoom")  == 0) { HandleHistoryZoom(json, slotIdx); }
     else if (strcmp(type, "historyInfo")  == 0) { HandleHistoryInfo(slotIdx); }
@@ -1017,6 +1018,12 @@ void StreamHub::HandleRearm() {
     HandleArm();
 }
 
+void StreamHub::HandleForceTrigger() {
+    rearmPending_ = false;
+    (void) trigger_.Force();
+    BroadcastTriggerState();
+}
+
 void StreamHub::HandleTrigStop(const char *json) {
     /* {"type":"trigStop","stopped":bool} — absent "stopped" toggles. */
     bool stopped = !trigger_.GetStopped();

Source/Applications/StreamHub/StreamHub.h

@@ -140,6 +140,7 @@ private:
     void HandleRearm();
     void HandleTrigStop(const char *json);
     void HandleSetTrigger(const char *json);
+    void HandleForceTrigger();
     void HandleZoom(const char *json, uint32 slotIdx);
     void HandleHistoryZoom(const char *json, uint32 slotIdx);
     void HandleHistoryInfo(uint32 slotIdx);

Source/Applications/StreamHub/TriggerEngine.cpp

@@ -14,6 +14,8 @@ TriggerEngine::TriggerEngine()
       stopped_(false),
       prevValue_(0.0),
       prevValid_(false),
+      lastTime_(0.0),
+      lastTimeValid_(false),
       trigTime_(0.0),
       firedPreSec_(0.0),
       firedPostSec_(0.0),
@@ -82,6 +84,11 @@ bool TriggerEngine::GetStopped() const {
 void TriggerEngine::CheckSample(float64 t, float64 v) {
     (void) mutex_.FastLock();
 
+    /* Track the newest watched timestamp in every state so Force() has a
+     * reference time to latch the capture window around. */
+    lastTime_      = t;
+    lastTimeValid_ = true;
+
     if (state_ != kTrigArmed) {
         mutex_.FastUnLock();
         return;
@@ -122,6 +129,25 @@ void TriggerEngine::CheckSample(float64 t, float64 v) {
     mutex_.FastUnLock();
 }
 
+bool TriggerEngine::Force() {
+    (void) mutex_.FastLock();
+
+    bool ok = lastTimeValid_ && (state_ != kTrigCollecting);
+    if (ok) {
+        state_        = kTrigCollecting;
+        trigTime_     = lastTime_;
+        firedPreSec_  = config_.windowSec * config_.prePercent / 100.0;
+        firedPostSec_ = config_.windowSec - firedPreSec_;
+        firedValid_   = true;
+        REPORT_ERROR_STATIC(MARTe::ErrorManagement::Information,
+                            "TriggerEngine: forced at t=%.6f (pre=%.4fs post=%.4fs)",
+                            trigTime_, firedPreSec_, firedPostSec_);
+    }
+
+    mutex_.FastUnLock();
+    return ok;
+}
+
 TrigState TriggerEngine::GetState() const {
     (void) mutex_.FastLock();
     TrigState ret = state_;

Source/Applications/StreamHub/TriggerEngine.h

@@ -106,6 +106,15 @@ public:
      */
     void CheckSample(float64 t, float64 v);
 
+    /**
+     * @brief Fire the trigger unconditionally at the most recent sample time of
+     * the watched signal, latching the pre/post window exactly as CheckSample
+     * does.  Any state except COLLECTING → COLLECTING.
+     * @return false when no sample has been seen yet, or a capture is already
+     * being collected.
+     */
+    bool Force();
+
     /** @return Current FSM state. */
     TrigState GetState() const;
 
@@ -127,6 +136,8 @@ private:
     bool          stopped_;
     float64       prevValue_;    ///< Last sample (edge detection)
     bool          prevValid_;    ///< First-sample guard in ARMED state
+    float64       lastTime_;     ///< Timestamp of the newest watched sample
+    bool          lastTimeValid_;///< true once a watched sample has been seen
     float64       trigTime_;     ///< Latched trigger time (Unix s)
     float64       firedPreSec_;  ///< Window pre-part latched at fire time
     float64       firedPostSec_; ///< Window post-part latched at fire time