Fix pane magnification causing mouse to drag tabs unexpectedly (#26383)

Previously, if a user clicked a button and moved the cursor out before
releasing, the click event was correctly prevented, but the pending
mouse-down state remained.
This caused unintended drags when the UI shifted due to magnification
settings.

Now, mouse-up clears the pending state:
- If over the button → clear state and trigger click handlers.
- If outside the button → clear state without triggering a click.

This avoids accidental drags while preserving expected click behavior.

Closes #24600

Release Notes:

- N/A

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Co-authored-by: Ben Kunkle <ben@zed.dev>