Window disappears after dragging and dropping

[xiejiang2014]

run demo
Set tool5 as a floating window,
Drag tool5 into tool6,
Expected:revert to initial state
Actual: tool5 & tool6 disappeared; tool7 & tool8 were abnormally positioned.

It cannot be reproduced every time

gif:

[ClxS]

I've had something similar to this occur as well. I've tracked it down to the changes introduced in the IsEmpty PR #298, locally reverting these changes stopped it for me.

In my particular case it wasn't even a floating window being redocked that caused it. With an Unreal Editor style dock layout, docking my top left Tool to the bottom quadrant of the bottom right pane would cause both of them to disappear. In my case it's a 100% reproduction.

[jl0pd]

It's easy to reproduce with Notepad sample, either

1. drag replace to be above find
2. drag find to be below replace, then drag replace to be below find

[BAndysc]

Fixed by #324