layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui_app.hpp

@@ -209,12 +209,17 @@ struct App {
   int layout_drag_row = -1;
   std::vector<float> layout_drag_start;
   /// Column drag state: which plot (right-hand cell), the two starting *plot* widths in logical px
-  /// (a cell's rect minus the y gutter only the first cell of a row has), and the cell weights
-  /// snapshot at drag begin (`layout_col_apply()` redistributes the pair's weight sum from these, so
-  /// a drag stays relative to where it started).
+  /// (each plot's `layout().axes.w`, the area the plotter places a cell by -- not the cell rect, which
+  /// adds the y and right gutters and the gap), and the cell weights snapshot at drag begin
+  /// (`layout_col_apply()` redistributes the pair's weight sum from these, so a drag stays relative to
+  /// where it started).
   int layout_col_drag_plot = -1;
   double layout_col_drag_left0 = 0.0;
   double layout_col_drag_right0 = 0.0;
+  /// The drag handle's x in the overlay's coordinates (logical px) when the drag began. The handle is
+  /// re-placed on the divider every frame, and a gesture reports its offset in the handle's own, moving,
+  /// coordinates; `layout_col_total_offset()` adds the handle's displacement since this back.
+  double layout_col_drag_handle_x0 = 0.0;
   std::vector<float> layout_col_drag_start;
   /// The editor's way out: one `Done` button in the plot area's top-right corner while
   /// the editor is up. Not part of `layout_buttons`, which is one button per row and per
@@ -2121,6 +2126,9 @@ GtkWidget* layout_col_handle(App& a, int plot);
 /// (the right-hand cell) begins with. False when `plot` has no left neighbour.
 bool layout_col_drag_begin(App& app, int plot);
 void layout_col_apply(App& app, int plot, double offset_px);
+/// A column drag's pointer travel from what its gesture reports: `offset` is relative to the handle, which
+/// has moved from `handle_begin` to `handle_now` (same units, the overlay's) since the drag began.
+double layout_col_total_offset(double offset, double handle_now, double handle_begin);
 void layout_probe_report(App& a, const char* what);
 /// The split in the report lines, from the `RowSet` alone (no widget is asked, so the exit dump can
 /// use them): cells per figure row (`1,2,1`) and each figure row's cell weights, rows divided by `|`