System Information
Windows 10 Pro x64 - 1703
GTX 970 (398.36 WHQL)

Blender Version
Broken: 2.8 1112f49
Worked: 2.8 a6340a5

Short description of error
Sidebar (N) > Open/Move/Close User Preferences Window Crashes to Desktop.

Exact steps for others to reproduce the error

1. Run Blender 2.8 1112f49
2. Press N to show Sidebar.
3. Expand View Object Types section. (Mouse over and scroll down to see entire list gives same result.)
4. Open > User Pref. Window. (Edit > User Preferences)
5. Move > User Pref. Window around a little. (Click & Drag on titlebar.)
6. Close > User Pref. Window. (With 'X' button on titlebar.)
7. Crashes to Desktop.

Edit: Reproducible even without Steps 2-3. Opening User Pref. Window, Click-Dragging it around and Closing it still Crashes to Desktop.

Tested

1. Tried this about 20 times. 16/20 times happens immediately. (Soon as I hit the 'X' to close user pref. window and move mouse.) Other couple of times it took a few tries.
2. Issue seems to be with moving around (click-drag) new windows? Tried, Window > New Window > Move window around > Press 'X' to close it. Crashed.
3. But seems more frequent with User Pref. Window. (More reproducible.)
4. It seems like a difficult crash to reproduce, but definitely crashes. And only started happening with this new commit. 1112f49

Please let me know if there is any more information I can provide to help. I don't have any screen-recording software installed or I would provide a video of the reproduction. But if required I can do it for you guys. :)