I did some more testing.....
From the Project viewer, just hitting ^F or using "Add Audio File" and causing it to open the file dialog, then not doing anything but hitting ESC to cancel, each time the window continuously grows wider.
However, if I use Shift-Cmd-I to open the same dialog, it is initially at whatever width the previous dialog was, however I am able to resize it normally. Subsequent Shift-Cmd-I windows stay the same size.
But the moment I "Add Audio File" from the project browser, the window begins to grow each iteration. Very odd, and very annoying.