[BUGFIX] Fix ContextMenu positioning race conditions

Read mouse position from the triggering event instead of relying
on values from a throttled mousemove event, which caused race
conditions where mouse position was not updated at all when moving
the mouse from iframe to outer frame.

Resolves: #103044
Releases: main, 12.4
Change-Id: If22aee7eea958969fb3e7e24b15523897b70e1ef
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83185


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>