[REG 5.10.0->5.11.1] Can't redock QDockWidget after floating, closing, and reopening

In Qt 5.11.1, you create a dock widget and it can become impossible to redock after floating.

To reproduce, run the attached project, drag the dock widget out of the main window so that it is floating.  At this time, the dialog is still allowed to be reattached (See attachAllowed image).

Then, hit the close button in the top right hand corner of the dialog. Make the dialog visible again via the right click context menu on the toolbar. Now, if you try to drag the docking widget back into any docking areas, it will not allow the docking (See attachUnallowed image).

I have tested this same program in Qt 5.10.0, and after closing and reopening, the docking widget would still dock correctly.