I regret the delay in getting back to you.
Running this sample with both the latest trunk and our production version of Lazarus (3.4), the behavior is the same:
Trying to dock the horizontally elongated window, there's super odd behavior.
As I approach the dock target form, the window "ghost" suddenly travels back roughly it's own size horizontally.
This happens all the while the cursor is still near my intended dock destination.
From that point onwards everything is very erratic and simply unusable.
Really disappointing to see that we still don't have working docking with Lazarus, even in its own out-of-the-box samples!