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

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