Updated local storage

CLAUDE.md

@@ -218,6 +218,17 @@ Two lines of the self-check output are assertions in disguise:
   floor are the two refusals, and because the probe emits `::clicked` straight at the
   widget it reaches the handler even when the button is insensitive, so `add above 0` there
   must leave `rows=` and `traces=` untouched rather than growing a ninth row.
+- `header: window_button_w=W label_w=L bar_w=B` (stderr, once a second) — **`W` must be
+  a small constant and must not track `B`**: the window control is a header-bar child,
+  and an expanding one is measured into the title's centre box, so it stretched from the
+  label to the title (516 px at a 1270 px bar before the fix, 121 px after). A `W` that
+  grows with the window width is that bug back.
+- **The session layout round trip** (`~/.config/udpscope/<host>_<port>.toml`) — run once
+  with a drop and a window change, then again with no interaction: the second run's dump
+  must read `layout loaded from ...` and `layout: rows=R window_s=W row0=[...]` with the
+  first run's values, and the signals it names must come back *acquired* (a `row 0 slot 0
+  -> channel ... samples=N` line). Delete the file before a run whose layout you did not
+  mean to inherit (`127.0.0.1_9540.toml` and friends accumulate otherwise).
 - `badge frames: lit=L dim=D idle=I` — the activity badge's phase per frame. Over a run
   that acquired anything **both `lit` and `dim` must be non-zero**: that is what "it
   blinks" means, and a badge stuck lit is indistinguishable from a working one in a

CMakeLists.txt

@@ -40,6 +40,7 @@ add_executable(udpscope
   src/acquisition.cpp
   src/plot_host.cpp
   src/export.cpp
+  src/layout_store.cpp
   src/ui.cpp
 )
 

README.md

@@ -46,6 +46,8 @@ src/acquisition.*  the UDPStreamer client, the producer thread, frame decoding
 src/trigger.*      the trigger configuration and the producer-side state machine
 src/plot_host.*    the one plot::Figure, its rows' traces and cursors, the x link group
 src/export.*       the visible range as a table, and the .npy / .csv writers
+src/layout_store.* the per-source session layout file (TOML), written and read back
+src/event_series.hpp  an immutable captured event as a plot::data::Series
 src/ui.*           the GTK4 window: toolbar, trigger panel, signal panel, plot box
 ```
 
@@ -56,6 +58,41 @@ frame and the GUI thread takes once per frame around binding the traces.
 Samples cross the boundary through `plot::data::RingBuffer` (single producer,
 single consumer), never through the lock.
 
+## The session layout file
+
+What you arranged is remembered per data source, in
+`~/.config/udpscope/<host>_<port>.toml` (`$XDG_CONFIG_HOME` is honoured; the
+host's unsafe characters are replaced with `_`). The file holds the display
+window, the row layout and which signal each of a row's slots plots, and the
+per-signal gain and offset:
+
+```toml
+window_s = 2
+plots = 1
+
+[[plot]]
+signals = ["Ch1", "", "", ""]
+
+[[calibration]]
+name = "Ch1"
+gain = 2
+offset = 0.5
+```
+
+It is written (400 ms after the last change, and flushed on close) whenever the
+window, the layout, a drop or a calibration changes, and applied once, when the
+first CONFIG names the signals -- signal *names* are the identity, because a
+CONFIG renumbers and an enable/disable rebuilds the channel list. Signals the
+file plots are enabled on load, since a restored plot nobody acquires would come
+back blank. It is plain TOML and safe to edit; a malformed file loads whatever
+parsed rather than refusing to start. Writes go to a sibling temp file and are
+renamed over the target, so a crash mid-write leaves the old layout.
+
+The same reason the window control is a fixed-width label in the header bar
+(see the DPI/relayout notes in `ui.cpp`) is why the bar's `window` control does
+not expand: a header-bar child that expands is measured into the title's centre
+box, so the control stretched to the title and every window resize moved it.
+
 ## How the spec maps onto the code
 
 **One buffer of `N+1` slices, plus frozen event snapshots.** `ScopeModel`

src/layout_store.cpp

@@ -0,0 +1,248 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "layout_store.hpp"
+
+#include <cstdint>
+#include <cstdio>
+#include <fstream>
+#include <sstream>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+namespace udpscope {
+
+namespace {
+
+/// Anything outside a safe filename set becomes '_': the host is a dotted quad
+/// today, but the CLI takes any string and a '/' or ':' in it must not escape
+/// the directory.
+std::string sanitize(const std::string& host) {
+  std::string out;
+  out.reserve(host.size());
+  for (char c : host) {
+    const bool ok = (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') ||
+                    (c >= '0' && c <= '9') || c == '.' || c == '_' || c == '-';
+    out.push_back(ok ? c : '_');
+  }
+  return out.empty() ? "_" : out;
+}
+
+std::string config_dir() {
+  const char* xdg = std::getenv("XDG_CONFIG_HOME");
+  if (xdg != nullptr && xdg[0] != '\0') return std::string(xdg) + "/udpscope";
+  const char* home = std::getenv("HOME");
+  if (home == nullptr || home[0] == '\0') return "/tmp/udpscope-config";
+  return std::string(home) + "/.config/udpscope";
+}
+
+void mkdirs(const std::string& path) {
+  std::string cur;
+  for (const char c : path) {
+    cur.push_back(c);
+    if (c == '/' || cur.size() == path.size()) {
+      if (cur == "/" || cur.back() != '/') continue;
+      ::mkdir(cur.c_str(), 0755);
+    }
+  }
+}
+
+std::string toml_escape(const std::string& s) {
+  std::string out = "\"";
+  for (char c : s) {
+    if (c == '"' || c == '\\') out.push_back('\\');
+    out.push_back(c);
+  }
+  return out + "\"";
+}
+
+bool unescape(std::string& s) {
+  std::string out;
+  for (std::size_t i = 0; i < s.size(); ++i) {
+    if (s[i] != '\\') {
+      out.push_back(s[i]);
+      continue;
+    }
+    if (++i >= s.size()) return false;
+    if (s[i] == '"' || s[i] == '\\') {
+      out.push_back(s[i]);
+    } else {
+      return false;
+    }
+  }
+  s = out;
+  return true;
+}
+
+std::string trim(const std::string& s) {
+  const auto a = s.find_first_not_of(" \t\r");
+  if (a == std::string::npos) return {};
+  const auto b = s.find_last_not_of(" \t\r");
+  return s.substr(a, b - a + 1);
+}
+
+/// `"a", "b"` (with the brackets already stripped) -> ["a", "b"].
+bool parse_string_array(const std::string& in, std::vector<std::string>& out) {
+  out.clear();
+  std::size_t i = 0;
+  while (i < in.size()) {
+    while (i < in.size() && (in[i] == ' ' || in[i] == ',' || in[i] == '\t')) ++i;
+    if (i >= in.size()) break;
+    if (in[i] != '"') return false;
+    ++i;
+    std::string tok;
+    while (i < in.size() && in[i] != '"') {
+      if (in[i] == '\\' && i + 1 < in.size()) {
+        tok.push_back(in[i]);
+        ++i;
+      }
+      tok.push_back(in[i]);
+      ++i;
+    }
+    if (i >= in.size()) return false;
+    ++i;  // closing quote
+    if (!unescape(tok)) return false;
+    out.push_back(tok);
+  }
+  return true;
+}
+
+}  // namespace
+
+std::string layout_path(const std::string& host, std::uint16_t port) {
+  const std::string dir = config_dir();
+  mkdirs(dir);
+  return dir + "/" + sanitize(host) + "_" + std::to_string(port) + ".toml";
+}
+
+bool load_layout(const std::string& path, LayoutState& out) {
+  std::ifstream in(path);
+  if (!in) return false;
+
+  LayoutState st;
+  st.window_s = 0.0;
+  st.rows.clear();
+  st.calibration.clear();
+
+  std::string line;
+  // What the last `[[...]]` header opened, so keys land in the right record.
+  enum class Section { Top, Plot, Calibration } section = Section::Top;
+  int plot_count = 0;
+  bool plot_section_seen = false;
+  while (std::getline(in, line)) {
+    const std::string t = trim(line);
+    if (t.empty() || t[0] == '#') continue;
+    if (t[0] == '[') {
+      if (t == "[[plot]]") {
+        section = Section::Plot;
+        plot_section_seen = true;
+        st.rows.emplace_back();
+      } else if (t == "[[calibration]]") {
+        section = Section::Calibration;
+        st.calibration.emplace_back();
+      } else {
+        section = Section::Top;  // an unknown table: keep its keys out of ours
+      }
+      continue;
+    }
+    const auto eq = t.find('=');
+    if (eq == std::string::npos) continue;
+    const std::string key = trim(t.substr(0, eq));
+    const std::string value = trim(t.substr(eq + 1));
+    auto strip_quote = [](const std::string& v) {
+      if (v.size() >= 2 && v.front() == '"' && v.back() == '"') {
+        std::string s = v.substr(1, v.size() - 2);
+        std::string out;
+        for (std::size_t i = 0; i < s.size(); ++i) {
+          if (s[i] == '\\' && i + 1 < s.size()) ++i;
+          out.push_back(s[i]);
+        }
+        return out;
+      }
+      return v;
+    };
+
+    if (section == Section::Top) {
+      if (key == "window_s") {
+        try {
+          st.window_s = std::stod(value);
+        } catch (...) {
+        }
+      } else if (key == "plots") {
+        // Only a fallback for a file with no [[plot]] sections: the writer emits
+        // both, and honouring both appended the sections onto the pre-created
+        // rows (a 2-plot file loaded as 4).
+        try {
+          plot_count = std::max(0, std::stoi(value));
+        } catch (...) {
+        }
+      }
+    } else if (section == Section::Plot && !st.rows.empty()) {
+      if (key == "signals") {
+        std::vector<std::string> names;
+        if (value.size() >= 2 && value.front() == '[' && value.back() == ']') {
+          parse_string_array(value.substr(1, value.size() - 2), names);
+        }
+        st.rows.back().signals = names;
+      }
+    } else if (section == Section::Calibration && !st.calibration.empty()) {
+      if (key == "name") {
+        st.calibration.back().name = strip_quote(value);
+      } else if (key == "gain") {
+        try {
+          st.calibration.back().gain = std::stod(value);
+        } catch (...) {
+        }
+      } else if (key == "offset") {
+        try {
+          st.calibration.back().offset = std::stod(value);
+        } catch (...) {
+        }
+      }
+    }
+  }
+  if (!plot_section_seen && plot_count > 0) {
+    st.rows.assign(static_cast<std::size_t>(plot_count), {});
+  }
+  out = std::move(st);
+  return true;
+}
+
+bool save_layout(const std::string& path, const LayoutState& state) {
+  std::ostringstream o;
+  o << "# udpscope session layout (written by the app; safe to edit)\n";
+  if (state.window_s > 0.0) o << "window_s = " << state.window_s << "\n";
+  o << "plots = " << state.rows.size() << "\n";
+  for (const auto& row : state.rows) {
+    o << "\n[[plot]]\nsignals = [";
+    for (std::size_t i = 0; i < row.signals.size(); ++i) {
+      if (i != 0) o << ", ";
+      o << toml_escape(row.signals[i]);
+    }
+    o << "]\n";
+  }
+  for (const auto& cal : state.calibration) {
+    o << "\n[[calibration]]\nname = " << toml_escape(cal.name) << "\ngain = " << cal.gain
+      << "\noffset = " << cal.offset << "\n";
+  }
+
+  // A sibling temp file + rename: a crash mid-write leaves the old layout, not a
+  // truncated one that would fail to parse next start.
+  const std::string tmp = path + ".tmp";
+  {
+    std::ofstream f(tmp, std::ios::trunc);
+    if (!f) return false;
+    f << o.str();
+    if (!f.good()) {
+      ::remove(tmp.c_str());
+      return false;
+    }
+  }
+  if (std::rename(tmp.c_str(), path.c_str()) != 0) {
+    ::remove(tmp.c_str());
+    return false;
+  }
+  return true;
+}
+
+}  // namespace udpscope

src/layout_store.hpp

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace udpscope {
+
+/// The user-arranged part of a session, persisted per data source in
+/// `~/.config/udpscope/<host>_<port>.toml`: the display window, the plot layout
+/// (rows and which signal each of a row's slots holds) and the per-signal
+/// calibration. Signal names are the stable identity (channel indexes are
+/// renumbered by every enable/disable, see the model's own invariant).
+struct LayoutState {
+  struct Row {
+    /// One entry per slot; an empty string is an empty slot.
+    std::vector<std::string> signals;
+  };
+
+  double window_s = 0.0;  ///< 0 keeps the default window
+  std::vector<Row> rows;
+
+  struct Calibration {
+    std::string name;
+    double gain = 1.0;
+    double offset = 0.0;
+  };
+  std::vector<Calibration> calibration;
+};
+
+/// `~/.config/udpscope/<host>_<port>.toml`, with the directory created.
+/// Characters unsafe in a filename are replaced in the host part.
+std::string layout_path(const std::string& host, std::uint16_t port);
+
+/// Parses the file's TOML subset. Returns false when the file is missing or
+/// malformed; `out` is left usable either way (a malformed file loads as much as
+/// parsed, which beats refusing a hand-edited file over one stray character).
+bool load_layout(const std::string& path, LayoutState& out);
+
+/// Writes `state` as TOML. Atomic in effect: writes a sibling temp file and
+/// renames it over the target.
+bool save_layout(const std::string& path, const LayoutState& state);
+
+}  // namespace udpscope

src/main.cpp

@@ -30,7 +30,7 @@ int main(int argc, char** argv) {
     return 1;
   }
 
-  const int result = udpscope::run_ui(argc, argv, model, acquisition, rows);
+  const int result = udpscope::run_ui(argc, argv, cfg, model, acquisition, rows);
   acquisition.stop();
   return result;
 }

src/model.cpp

@@ -184,6 +184,12 @@ void ScopeModel::rebuild_channels() {
     ch.unit = sig.unit;
     ch.elements = sig.elements();
     ch.dt = sig.sampling_rate > 0.0 ? 1.0 / sig.sampling_rate : 0.0;
+    // A calibration the user already set for this name (before it was enabled, or
+    // before a CONFIG renumbered the channels) still applies.
+    if (auto it = calibration_.find(sig.name); it != calibration_.end()) {
+      ch.gain = it->second.first;
+      ch.offset = it->second.second;
+    }
     channels_.push_back(std::move(ch));
   }
 }
@@ -277,6 +283,7 @@ void ScopeModel::set_signal_enabled(std::size_t signal, bool on) {
 
 void ScopeModel::set_calibration(std::size_t signal, double gain, double offset) {
   std::lock_guard<std::mutex> lock(mutex_);
+  if (signal < signals_.size()) calibration_[signals_[signal].name] = {gain, offset};
   bool any = false;
   for (ChannelInfo& c : channels_) {
     if (c.signal != signal) continue;

src/model.hpp

@@ -11,6 +11,7 @@
 #include <plot/data/ring_buffer.hpp>
 
 #include <atomic>
+#include <unordered_map>
 #include <cstdint>
 #include <memory>
 #include <mutex>
@@ -183,6 +184,11 @@ class ScopeModel {
   static bool is_time_signal(const std::vector<SignalInfo>& signals,
                              std::size_t signal);
   void set_signal_enabled(std::size_t signal, bool on);
+  /// Stores the calibration for a signal's *name* and applies it to that signal's
+  /// live channels. Keyed by name, not index: a CONFIG renumbers and an
+  /// enable/disable rebuilds the channel list, and `rebuild_channels()` re-applies
+  /// from here -- so a calibration set before a signal was enabled, or surviving a
+  /// CONFIG, is not silently dropped.
   void set_calibration(std::size_t signal, double gain, double offset);
 
   std::size_t channel_count() const noexcept { return channels_.size(); }
@@ -352,6 +358,8 @@ class ScopeModel {
   std::atomic<bool> trigger_pending_{false};
   std::atomic<double> capture_edge_{0.0};
   std::atomic<std::shared_ptr<const EventSnapshot>> event_;  ///< the event on screen
+  /// Per-signal-name calibration (see `set_calibration`).
+  std::unordered_map<std::string, std::pair<double, double>> calibration_;
   std::atomic<std::uint64_t> event_seq_{0};
   std::atomic<double> display_epoch_{0.0};
   std::atomic<bool> trigger_armed_{false};

src/ui.cpp

@@ -17,6 +17,7 @@
 #include "acquisition.hpp"
 #include "config.hpp"
 #include "export.hpp"
+#include "layout_store.hpp"
 #include "model.hpp"
 #include "plot_host.hpp"
 
@@ -121,6 +122,13 @@ struct App {
   GtkWidget* cursors_toggle = nullptr;
   GtkWidget* cursors_reset = nullptr;
 
+  /// The persisted-layout file this data source maps to (--server/--port), a
+  /// debounced save handle, and whether the file has been applied once the
+  /// CONFIG arrived.
+  std::string source_host;
+  std::uint16_t source_port = 0;
+  guint layout_save_source = 0;
+  bool layout_loaded = false;
   unsigned signal_generation = ~0u;
   bool updating_panels = false;  ///< guards signal handlers during a rebuild
   bool relayout_pending = false;
@@ -292,6 +300,121 @@ GtkWidget* labeled_box(const char* label, GtkWidget* child) {
   return box;
 }
 
+/// Same, without expanding the child. Header-bar children expand through the
+/// title's centre box (a start-packed expanding widget is measured into it), so
+/// the window control there stretched all the way to the title; panels want the
+/// fill, the bar does not.
+// --- persisted layout -------------------------------------------------------
+
+void apply_time_window(App& app, double seconds);
+int signal_color_index(const std::vector<SignalInfo>& signals, std::size_t signal);
+void rebuild_signal_panels(App& app);
+
+/// Reads the session layout back out of the model and rows: the display window,
+/// each row's slot signal names, and each signal's calibration.
+LayoutState collect_layout(App& app) {
+  LayoutState st;
+  st.window_s = app.rows->time_window();
+  ScopeModel& model = *app.model;
+  for (int i = 0; i < app.rows->row_count(); ++i) {
+    LayoutState::Row row;
+    const RowSet::Row& r = app.rows->row(i);
+    for (int s = 0; s < RowSet::kSlotsPerRow; ++s) {
+      row.signals.push_back(r.signal_name[static_cast<std::size_t>(s)]);
+    }
+    st.rows.push_back(std::move(row));
+  }
+  for (std::size_t si = 0; si < model.signals().size(); ++si) {
+    for (std::size_t c = 0; c < model.channels().size(); ++c) {
+      const ChannelInfo& ch = model.channels()[c];
+      if (ch.signal != si) continue;
+      st.calibration.push_back({model.signals()[si].name, ch.gain, ch.offset});
+      break;
+    }
+  }
+  return st;
+}
+
+/// Writes the layout file (a debounced schedule lands here once). Errors go to
+/// stderr; success is silent -- a save per click must not narrate.
+void save_layout_now(App& app) {
+  if (app.source_port == 0) return;
+  if (!save_layout(layout_path(app.source_host, app.source_port), collect_layout(app))) {
+    std::fprintf(stderr, "[udpscope] failed to write the layout file\n");
+    std::fflush(stderr);
+  }
+}
+
+/// One-shot debounce: a burst of changes (a drop, a row insert, a calibration
+/// commit in the same second) writes once, 400 ms after the last one.
+gboolean layout_save_tick(gpointer user) {
+  App* app = static_cast<App*>(user);
+  app->layout_save_source = 0;
+  save_layout_now(*app);
+  return G_SOURCE_REMOVE;
+}
+
+void schedule_layout_save(App& app) {
+  if (app.source_port == 0) return;
+  if (app.layout_save_source != 0) return;
+  app.layout_save_source = g_timeout_add(400, layout_save_tick, &app);
+}
+
+/// Applies a loaded layout: rows, slot assignments, calibration, window. Names
+/// that no longer exist in the stream are skipped rather than freeing their slot
+/// (a stream that renamed a signal must not blank the user's layout).
+void apply_layout(App& app, const LayoutState& st) {
+  ScopeModel& model = *app.model;
+  const auto& signals = model.signals();
+
+  int rows = static_cast<int>(st.rows.size());
+  if (rows > RowSet::kMaxRows) rows = RowSet::kMaxRows;
+  if (rows > 0) app.rows->set_row_count(rows);
+
+  for (int i = 0; i < rows; ++i) {
+    int slot = 0;
+    for (const std::string& name : st.rows[static_cast<std::size_t>(i)].signals) {
+      if (slot >= RowSet::kSlotsPerRow) break;
+      if (name.empty()) continue;
+      for (std::size_t si = 0; si < signals.size(); ++si) {
+        if (signals[si].name != name) continue;
+        // Enabling is what starts acquisition (a loaded plot nobody acquires would
+        // come back blank), and it is what a drop does for the signal it places.
+        app.rows->assign(i, slot, name, name, signal_color_index(signals, si));
+        model.set_signal_enabled(si, true);
+        ++slot;
+        break;
+      }
+    }
+  }
+
+  for (const auto& cal : st.calibration) {
+    for (std::size_t si = 0; si < signals.size(); ++si) {
+      if (signals[si].name != cal.name) continue;
+      model.set_calibration(si, cal.gain, cal.offset);
+      break;
+    }
+  }
+
+  if (st.window_s > 0.0) apply_time_window(app, st.window_s);
+
+  // set_row_count() rebuilt the figure (its row count is fixed at construction):
+  // re-point the area, as every other row-count change must, and refresh the
+  // signal panel's chips so they name the traces the file brought back.
+  if (app.area != nullptr) app.area->set_figure(&app.rows->figure());
+  app.relayout_pending = true;
+  rebuild_signal_panels(app);
+}
+
+GtkWidget* labeled_box_fixed(const char* label, GtkWidget* child) {
+  GtkWidget* box = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 6);
+  GtkWidget* l = gtk_label_new(label);
+  gtk_widget_set_halign(l, GTK_ALIGN_START);
+  gtk_box_append(GTK_BOX(box), l);
+  gtk_box_append(GTK_BOX(box), child);
+  return box;
+}
+
 GtkWidget* spin(double min, double max, double step, int digits) {
   GtkWidget* s = gtk_spin_button_new_with_range(min, max, step);
   gtk_spin_button_set_digits(GTK_SPIN_BUTTON(s), digits);
@@ -646,6 +769,7 @@ void on_layout_row_remove(GtkButton* button, gpointer user) {
   // The signal list's chips name the trace a signal is drawn as, and a removed row took
   // its traces with it.
   rebuild_signal_panels(*app);
+  schedule_layout_save(*app);
 }
 
 void on_layout_row_insert(GtkButton* button, gpointer user) {
@@ -659,6 +783,7 @@ void on_layout_row_insert(GtkButton* button, gpointer user) {
   if (app->area != nullptr) app->area->set_figure(&app->rows->figure());
   rebuild_layout_editor(*app);
   rebuild_signal_panels(*app);
+  schedule_layout_save(*app);
 }
 
 /// Restates where the buttons sit, from the rects the last frame was laid out with.
@@ -875,6 +1000,7 @@ void on_calibrate_apply(GtkButton*, gpointer user) {
   CalibContext* ctx = static_cast<CalibContext*>(user);
   ctx->app->model->set_calibration(ctx->signal, gtk_spin_button_get_value(GTK_SPIN_BUTTON(ctx->gain)),
                                    gtk_spin_button_get_value(GTK_SPIN_BUTTON(ctx->offset)));
+  schedule_layout_save(*ctx->app);
   gtk_window_destroy(GTK_WINDOW(ctx->window));
 }
 
@@ -1151,6 +1277,7 @@ void apply_time_window(App& app, double seconds) {
   app.rows->set_time_window(clamped);
   app.model->set_event_span(clamped);
   sync_window_display(app, clamped);
+  schedule_layout_save(app);
 }
 
 /// Enter in the entry. A number that will not parse restores the current window instead of
@@ -1575,6 +1702,7 @@ gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y
   const std::vector<SignalInfo>& signals = app->model->signals();
   const SignalInfo& sig = signals[signal];
   app->rows->assign(row, slot, sig.name, name, signal_color_index(signals, signal));
+  schedule_layout_save(*app);
   app->relayout_pending = true;
   rebuild_signal_panels(*app);  // the row's colour chip now has a trace to name
   cancel_drag_on(*app, row);
@@ -1656,8 +1784,37 @@ gboolean tick(gpointer user) {
     app->relayout_pending = false;
     relayout_areas(*app);
   }
+  // Header geometry, once a second: the window control must be a fixed, small
+  // widget beside the title whatever the bar's width is (it used to expand through
+  // the title's centre box, i.e. window_button_w tracked the window width).
+  if (app->window_button != nullptr) {
+    static gint64 next_header_log = 0;
+    const gint64 now = g_get_monotonic_time();
+    if (now >= next_header_log) {
+      next_header_log = now + G_TIME_SPAN_SECOND;
+      std::fprintf(stderr, "[udpscope] header: window_button_w=%d label_w=%d bar_w=%d\n",
+                   gtk_widget_get_width(app->window_button),
+                   gtk_widget_get_width(app->window_label),
+                   gtk_widget_get_width(app->window));
+      std::fflush(stderr);
+    }
+  }
   if (model.generation() != app->signal_generation) rebuild_signal_panels(*app);
 
+  // The first CONFIG names the signals, which is what the persisted layout is
+  // written against: apply it once, here, and never again (a later CONFIG only
+  // renames or renumbers; the user's layout survives those untouched).
+  if (!app->layout_loaded && !model.signals().empty()) {
+    app->layout_loaded = true;
+    LayoutState st;
+    if (load_layout(layout_path(app->source_host, app->source_port), st)) {
+      apply_layout(*app, st);
+      std::fprintf(stderr, "[udpscope] layout loaded from %s\n",
+                   layout_path(app->source_host, app->source_port).c_str());
+      std::fflush(stderr);
+    }
+  }
+
   // What the display shows, decided once for the whole frame: the producer can open or
   // close a capture between any two reads of the model, and the bind, the epoch, the pin
   // and the status line all have to describe the same picture.
@@ -2255,7 +2412,7 @@ GtkWidget* build_toolbar(App& app) {
   GtkWidget* bar = gtk_header_bar_new();
 
   gtk_header_bar_pack_start(GTK_HEADER_BAR(bar),
-                            labeled_box("window", build_window_control(app)));
+                            labeled_box_fixed("window", build_window_control(app)));
   refresh_window_limits(app);
 
   // --- the overflow menu ----------------------------------------------------
@@ -2348,7 +2505,15 @@ void on_activate(GtkApplication* application, gpointer user) {
   // A window closed by hand (a WM close button) must stop the frame tick: it holds
   // pointers to widgets that are finalized with the window.
   g_signal_connect(app->window, "destroy", G_CALLBACK(+[](GtkWidget*, gpointer user) {
-                     static_cast<App*>(user)->window = nullptr;
+                     App* a = static_cast<App*>(user);
+                     // A pending debounced save will never fire now: flush it, so
+                     // quitting right after a change keeps the change.
+                     if (a->layout_save_source != 0) {
+                       g_source_remove(a->layout_save_source);
+                       a->layout_save_source = 0;
+                       save_layout_now(*a);
+                     }
+                     a->window = nullptr;
                    }),
                    app);
   gtk_window_set_title(GTK_WINDOW(app->window), "udpscope");
@@ -3078,12 +3243,16 @@ gboolean exit_probe_step1(gpointer user) {
 
 }  // namespace
 
-int run_ui(int argc, char** argv, ScopeModel& model, Acquisition& acq, RowSet& rows) {
+int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisition& acq,
+           RowSet& rows) {
   (void)argc;
   App app;
   app.model = &model;
   app.acq = &acq;
   app.rows = &rows;
+  // Which data source this session talks to: the persisted-layout file's name.
+  app.source_host = cfg.server;
+  app.source_port = cfg.port;
 
   GtkApplication* application =
       gtk_application_new("org.udpscope.app", G_APPLICATION_DEFAULT_FLAGS);
@@ -3240,6 +3409,21 @@ int run_ui(int argc, char** argv, ScopeModel& model, Acquisition& acq, RowSet& r
                     tr.series->x_bounds().max);
       }
     }
+    std::printf("[udpscope]   header: window_button_w=%d window_label_w=%d (bar_w=%d)\n",
+                app.window_button != nullptr ? gtk_widget_get_width(app.window_button) : -1,
+                app.window_label != nullptr ? gtk_widget_get_width(app.window_label) : -1,
+                app.window != nullptr ? gtk_widget_get_width(app.window) : -1);
+    std::printf("[udpscope]   layout: rows=%d window_s=%.6g", rows.row_count(),
+                rows.time_window());
+    for (int r = 0; r < rows.row_count() && r < RowSet::kMaxRows; ++r) {
+      std::printf(" row%d=[", r);
+      for (int s = 0; s < RowSet::kSlotsPerRow; ++s) {
+        if (s != 0) std::printf(",");
+        std::printf("'%s'", rows.row(r).signal_name[static_cast<std::size_t>(s)].c_str());
+      }
+      std::printf("]");
+    }
+    std::printf("\n");
     for (int r = 0; r < rows.row_count() && r < RowSet::kMaxRows; ++r) {
       for (int s = 0; s < RowSet::kSlotsPerRow; ++s) {
         const int ch = rows.channel_of(r, s);

src/ui.hpp

@@ -12,6 +12,7 @@ class RowSet;
 
 /// Runs the GTK application. Never returns while the window is open; returns the
 /// `g_application_run()` result once it closes.
-int run_ui(int argc, char** argv, ScopeModel& model, Acquisition& acq, RowSet& rows);
+int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisition& acq,
+           RowSet& rows);
 
 }  // namespace udpscope