[rdrake]

Selecting text in a modal and letting go past its edge closed the modal, along with anything typed in it.

Why

When the mouse button goes down on one element and up on another, the browser fires the click on their nearest common ancestor. For anything inside a modal, that ancestor is the backdrop. @click.self can't tell such a click from a real one on the backdrop. Two cases closed the modal:

• a drag that started inside the dialog and was released over the backdrop;
• a press on the backdrop that was released inside the dialog.

The worst cases are the ban dialog and the alert builder, where the text the user typed is lost. The script editor, Mentions, the context menu and every settings-style window behaved the same way. Touch isn't affected, because a touch drag fires no click.

What

• v-backdrop directive (client/js/helpers/backdrop.ts, registered in vue.ts). It counts a click only when the pointer went down and came up on the backdrop itself, which is how a native <dialog>'s light dismiss behaves.
  • It replaces every @click.self on an overlay, and the click half of Mentions' and the context menu's own handlers.
  • Their remaining right-click-to-close is now @contextmenu.self.
  • test/tests/backdrop.ts fails if @click.self comes back.
• Not native <dialog>: closedby isn't supported on iOS Safari, and moving to <dialog> would break the escape-layer ordering and the overlays' z-index scheme.
• Scenario helper: scenarios now close a backdrop with clickBackdrop (tools/scenarios/lib/backdrop.mjs), a real mouse press and release. A bare element.click() from script no longer closes anything, so help-modal, inbox and native-shell-chrome were updated.

Testing

• test/helpers/backdrop.ts covers the directive's rules. The full mocha suite passes.
• New browser check tools/scenarios/backdrop-drag.mjs passes 7/7. On the old code the drag-out and press-in cases fail.
• help-modal and inbox pass. native-shell-chrome fails one check ("the page is itself again with the shell off"), which fails the same way on develop.
• Headless Chrome makes no text selection from scripted mouse events, so the drag check confirms the drag's click lands on the backdrop rather than checking a selection.

Mouse and trackpad only, so no phone testing is needed for the fix itself. The second commit is the review round; squash on merge if you prefer.