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

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