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()` /