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>

docs/claude/invariants-gtk-and-chrome.md

@@ -51,12 +51,21 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   how the first version of that line earned them. What stays in the menu is what acts on the *data*
   (Fit, Export, Configurations) and the one editor that is not a pane (the layout editor): the signal list
   is the `signals` slide-over page, so there is nothing left to reveal or hide about it. The Arm/Disarm button in the trigger panel still has text.
-- **Five of this app's icons are its own, and they are a `GResource` compiled into the
+- **An overlay handle that follows what it drags reports gesture offsets in its own moving
+  coordinates; add its displacement back.** A `GtkGestureDrag` measures its offset from where the
+  press began *in the widget's own coordinates*. The layout editor's column handle sits on the
+  divider and `sync_layout_editor()` moves it every frame, so after a few motion events the
+  offset is the pointer's travel **minus** the handle's travel, and the divider converges on half
+  the pointer's travel (`D = P - D`). Record the handle's x in the overlay's coordinates at
+  begin (`gtk_widget_compute_point`) and add `handle_now - handle_begin` to the offset on every update
+  (`layout_col_total_offset()`). The same applies to any future handle that is repositioned
+  from the state its own drag changes.
+- **Six of this app's icons are its own, and they are a `GResource` compiled into the
   binary.** No icon theme has a glyph for an oscilloscope trigger, a pair of cursors or a
   measurement ruler, and the stock names those buttons used to carry said something else
   entirely: a *record dot* for the trigger and three different **magnifiers** for cursors,
-  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows}-symbolic.svg`
-  are the five action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
+  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows,resize-cols}-symbolic.svg`
+  are the six action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
   user; `udpscope-measure-symbolic` **stayed**, because the Preferences window's Plot page carries it --
   an icon with a live user is not an orphan, whatever the toggle that introduced it did -- and it is
   the Preferences window's icon and the **Dock toggle's** (the ruler: the dock is the readouts).