I've seen this behavior with some weird third-party software (ones that "enhance" Finder).
Did you check behind the sidebar?
Also, do you see that behavior with any other software? What about opening workbook via File > Open or connecting to text/Excel files?
Tableau uses operating system's default FileSave and FileOpen dialogs so it's weird that you see that.
+1 same exact behavior on Mac Mojave with Tableau 2019.1 (but had the issue with 2018.3 as well).
I noticed Tableau starts struggling if i close my notebook without closing the application, then reopen the notebook (usually the following day) and try to keep working.
Any action that opens a finder window behaves like Nicholas Garcia describes after that. I also wondered whether it's related to using multiple screens, but that's just a hunch.
If i force quit Tableau and restart, the dialog works again.
+1 Same behaviour! Also on MacBook Pro (2018 with Mojave. working with 2019.1). I can open a new book, fake a couple of worksheets in it, and save that one. But the book I've just put a lot of work into has the exact same flickering behaviour.
If I try to close, I still get the "Do you want to save this workbook?" prompt, but get the same immediately disappearing save window.