run htop or btm and you can't select terminal text with shift+drag, the
usual escape hatch does nothing.

#25143 made shift+click only ever extend a selection, never start one.
so with nothing selected there's no anchor and the drag has nothing to
grab. now it drops an anchor when there's no selection, and still
extends when there is.

added two event-level tests (harness can't sync without a window):
shift+drag under mouse tracking, which fails without the fix, and
shift+click still extending.

Fixes #60254.

Release Notes:

- Fixed Shift+click-drag not selecting terminal text while an app has
mouse tracking enabled