Add an explicit overlay unfocus target so callers can move input to the editor or another component while overlays remain visible. Align fallback overlay focus with visual focus order and cover blocked replacement release, null targets, and multi-overlay cycling.
Preserve a focused visible overlay as the input owner across extension custom UI replacement, while letting the replacement receive and close its own input before focus is restored. Fixes#5129.
Tests assumed renders complete synchronously after requestRender() +
nextTick + flush(), but commit 6f5f37f8 changed requestRender() to
schedule via setTimeout with a 16ms minimum interval. Tests reading
viewport state before doRender() runs saw stale data.
Add VirtualTerminal.waitForRender() helper that waits for the throttled
render to settle (nextTick + 20ms + xterm flush), and replace all
terminal.flush() calls after render-triggering actions with it.
Fixes 8 failing tests across overlay-short-content, tui-render, and
markdown test suites.
