Proposed solution for issue #39924 - AnchorDocking Header position resets

A couple of days ago, Andrey Zubarev provided a patch (AnchorDocking.diff) to fix issue #39924 (closed).

I tried out the patch on a clean install of the 2.2.2 and the main Lazarus branches and can confirm that it fixes the issue.

It would be great if these changes could be merged back into the Lazarus code base, hence this merge request.