[bug] Windows start floating when Popup opens immediate

[X39]

I have a very fancy bug that i just discovered due to a popup opening in a buggy way.

Preparations

1. Create a new project with AvalonDock
2. Create two top-aligned anchorables
3. Make a popup open, the moment the anchorable gets focus Popup has to be sized above the actual tab you just pressed

Steps to Reproduce

1. Click the tab where the popup opens
2. Move your mouse cursor

Issue

2. Anchorable starts getting dragged

Expected

2. Anchorable should not get dragged