Issues working with multiple windows (Mac)

Observed on MacOS, 1.0alpha2 (e9298ea, 2019-07-05)

Certain types of window usage may result in unclosable windows, partial loss of Inkscape functionality, and/or freezing when trying to close documents or quit.

One possible set of steps to reproduce:

1. Open Inkscape 1.0
2. Open Preferences window (e.g., by command-bar button)
3. File > New
4. Make an object on the page (say, an ellipse)
5. Open (another) Preferences window
6. Close the first document

Now, there are two Preferences windows, and neither can be closed.

If you try to quit (File > Quit), you get a "Save document?" dialog, but it freezes if you select "Close without saving."

Various combinations of similar actions can result in freezing when trying to close a document (before getting to choose to save or not), and parts of the UI becoming inoperative. It may be worth trying to debug one specific case like that listed above before digging in to try all possible combinations.