Pressing the mouse inside a dialog (e.g. to select text in an input), dragging
out, and releasing on the backdrop wrongly dismissed it: the click event fires
on the common ancestor of mousedown+mouseup (the backdrop), so the outside-click
handler ran. Track whether the press started on the backdrop and only close when
it both starts and ends there. Fixes all shared-Modal dialogs at once (downloads
editor/share, confirm, Plex collections + add-to-playlist, …).
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