Reposition dialogs when their size changes instead of polling every 500ms

Problem/Motivation

Since #3252978: Large modal exceeds viewport height without scrollbar = actions unreachable, a setInterval() checks every 500ms if a dialog goes below the bottom of the window. If it does, it makes the dialog smaller and moves it to the center. So when a form gets bigger, the dialog moves up to 500ms later. A click at the moment the dialog moves can hit the overlay instead.

This makes our Behat tests fail at random: about 1 in 200 clicks on the dialog's Save button was lost (which made this issue very hard to trace).

Also, once the dialog is made smaller, it keeps that fixed height. jQuery UI ignores maxHeight when height is a number, so core's resizing no longer applies.

modal-height.cy.js does not find this problem. It waits 500ms, and should('be.visible') does not check the window. The test passes even with repositioning disabled.

Steps to reproduce

- Use a window that is not very tall.
- Open a component form and make it grow until the dialog goes below the bottom of the window, for example by expanding the Behaviors details element (present when the paragraph type has behavior plugins) or by letting CKEditor load.
- Part of the dialog stays off-screen. Up to 500ms later, the dialog jumps up.

Proposed resolution

- Use a ResizeObserver to move the dialog when its size changes, with no polling.
- Limit the height with maxHeight instead of a fixed height, so core can still resize the dialog.
- Update modal-height.cy.js so it fails without the fix.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.

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Debug and report assisted with the help of an LLM.