layout editor: in-between split buttons sit above the divider handle; doc nits

docs/claude/invariants-plot-and-layout.md

@@ -126,7 +126,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   - **One y per figure row.** The cells of a figure row share one y range (the plotter's
     `Figure::set_share_row_y(true)`, which `rebuild_all()` sets before the cells are populated): one y
     axis (auto or fixed), one pair of horizontal rulers, one fit. A wheel or axis drag in any cell moves
-    the row, an x gesture on any bottom-row cell moves the shared window. The host therefore acts on the
+    the row, an x gesture on any cell that carries an x strip moves the shared window. The host therefore acts on the
     **figure row** of a plot it is given (`set_y_auto`, `set_y_range`, `fit_all`, the Auto/Fixed dialog,
     `row_unit`, which joins the units of every cell and reads "" if they differ). The rulers are stored on
     the row's first plot index, copied to the other cells by `sync_cursors()` / `insert_cell()` /

docs/claude/invariants-time-axis-and-window.md

@@ -74,7 +74,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   slightly around when user zoom/pan" was. Three things follow. The **text is the host's**, because
   the caption has to name the anchor precisely enough that label + caption is the exact absolute
   time and `format_time()` would round it to three figures (`"t0 = %.6g s"`); only a plot
-  whose x strip draws labels** (`plot::x_strip_has_labels(x_strip_mode())`: the bottom row, and any row above it whose
+  whose x strip draws labels (`plot::x_strip_has_labels(x_strip_mode())`: the bottom row, and any row above it whose
   columns differ from the row below's) carries one, every other plot being set to `""`; and the renderer measures it
   rather than `Plot::layout()` — it reserves no space and nothing can be picked on it, so there is
   no rect to read back, and a caption explaining the x labels must not be able to move the layout

docs/claude/verification.md

@@ -206,7 +206,7 @@ precedent, not an exception.
   emitting `::clicked`, never `gtk_widget_activate()`, which holds a `GtkButton` for its
   250 ms `ACTIVATE_TIMEOUT` and then emits into a button the rebuild has already destroyed.
 - `UDPSCOPE_PROBE_SPLIT_MS` — drive the horizontal plot split, 28 steps 250 ms apart (so a run needs
-  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=10500` at least; it enables and assigns the
+  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=12000` at least; it enables and assigns the
   signals itself, so `UDPSCOPE_ENABLE_ALL_MS` is not needed). Each step restructures the cells,
   re-points the area at the new figure and lets the *next* step read a frame that was actually laid out
   with them, which is why every report line describes the previous step's change. It splits and

src/ui.cpp

@@ -666,7 +666,10 @@ void sync_layout_editor(App& app) {
       gtk_widget_set_margin_start(
           b, static_cast<int>(std::clamp(cx, 0.0,
                                          std::max(0.0, static_cast<double>(gtk_widget_get_width(app.area_widget)) - kLayoutButtonPx)) + 0.5));
-      gtk_widget_set_margin_top(b, static_cast<int>(std::clamp(row_cy - half, 0.0, height - kLayoutButtonPx) + 0.5));
+      // An in-between `+` sits above the divider's resize handle (centred on the same point), or the
+      // handle, added later, would cover it and "split here" there could not be clicked.
+      const double top = pos > 0 && pos < n ? row_cy - half - kLayoutButtonPx - 2.0 : row_cy - half;
+      gtk_widget_set_margin_top(b, static_cast<int>(std::clamp(top, 0.0, height - kLayoutButtonPx) + 0.5));
     } else if (resize) {
       // Row resize handle: placed on the boundary between figure rows.
       const int idx = row_key(b);