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.