Hiding a TextArea while mouse-based selection is happening can trap the cursor

[davep]

Initially discovered and reported in a comment in #4274 by @TomJGooding

Somewhat following on from #4274, although very likely an easier fix (I think it just needs a Hide handler to release the mouse and do any other housekeeping necessary to TextArea selection), if you start a mouse selection within TextArea and then hide the TextArea, the mouse will be trapped and won't respond anywhere else.

To illustrate:

from pathlib import Path
from textual.app import App, ComposeResult
from textual.widgets import Collapsible, TextArea


class GreedyScrollbarApp(App[None]):
    BINDINGS = [("f1", "collapse")]

    CSS = """
    TextArea {
        height: 10;
    }
    """

    def compose(self) -> ComposeResult:
        for n in range(5):
            with Collapsible(title=f"This is collapsible #{n}", collapsed=bool(n)):
                yield TextArea(Path(__file__).read_text(), language="python")

    def action_collapse(self) -> None:
        target.collapsed = not (target := self.query(Collapsible).first()).collapsed


if __name__ == "__main__":
    GreedyScrollbarApp().run()

Start a mouse selection in the first TextArea, press F1 while still selecting with the mouse, then notice nothing else will respond to mouse clicks.

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