On VS Code's xterm.js the transcript is taller than the viewport, so an
in-place clear (home + erase-to-end) only wipes the visible rows and the
scrolled-away part lingers in retained scrollback, stacking a duplicate
on the next full repaint.
- Clear() (Ctrl+L) now emits \x1b[3J under keepScrollback to actually
drop that scrollback, then homes and repaints. Accepts VS Code's
viewport-snap since the user explicitly asked for a clean screen.
- Overlay close (esc on a dialog, slash/file popup dismissal) now runs
the same Clear() so closing a picker purges the stale overlay rows
instead of leaving them in scrollback.
- Resize() does the same purge under keepScrollback; previously it
skipped the wipe and left a half-repainted old-width frame until the
user pressed Ctrl+L.
Other terminals keep their no-snap clear path.
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