fix: set relatedTarget when the pointer moves between elements

[Om-singhaI]

What

relatedTarget was always null on mouseover, mouseout, mouseenter, mouseleave and the matching pointer events. This sets it to the element the pointer moved to (for out and leave) or came from (for over and enter).

Fixes #1187, fixes #1182. This should also fix #1154.

Why

React reads relatedTarget on mouseout to work out which components get onMouseLeave. With null it assumes the pointer left the page. So if you hover a parent and click a child that only renders while the parent is hovered, the parent's onMouseLeave runs, the child unmounts and the click never lands. The workarounds in the thread are skipHover, which skips the pointer move inside user.click, or fireEvent.click.

How

Mouse.move and Pointer.move now pass nextTarget as relatedTarget for out and leave, and prevTarget for over and enter. If prevTarget was removed from the document it stays null. React ignores a mouseover whose relatedTarget is a node it rendered, and the removed node's mouseout never reaches React, so onMouseEnter would be lost. The first move still starts from document.body, so relatedTarget is body there. Touch Pointer.init and Pointer.release still use null.

Tests:

• React tests in tests/react/index.tsx for the hover then click children lost target #1187 case and for hovering after the previous element was removed
• relatedTarget checks for mouse and pointer events in tests/pointer/move.ts and tests/convenience/hover.ts

npx jest tests/pointer tests/convenience tests/system tests/react/index.tsx passes all 100 tests. With the source change reverted, the 4 tests for this fail and 96 pass. Without the removed element check, its 2 tests fail. I also wrote small tests for the #1182 example and the case described in #1154 (not in this PR). Both fail on main and pass with this change. No event log snapshots changed.

Checklist

• N/A Documentation
• Tests
• Ready to be merged