Real-browser regression coverage for the previous commit:
- SplitTerminalPane connects onto an already-quiet session and shows its
existing scrollback with no new output, proving the ?full=1 fetch (not
a live echo) populated the pane.
- openSplitPane() force-resizes Pane A synchronously as part of opening
a split.
- Dragging the divider force-resizes Pane A once, at drag end.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>