Some windows disappear when zooming. The most common windows to disappear are Firefox and xfce4-panel. It doesn't always happen immediately, but always happens after a couple of seconds of zooming in and out and panning around. The windows reappear after zooming all the way out back to the normal desktop view.

I have bisected it back to somewhere between these commits:

commit 11493fd453481b4c832986b1123160435b53a27a (refs/bisect/bad)
Author: Olivier Fourdan <fourdan@xfce.org>
Date:   Sun May 12 12:03:18 2019 +0200

    Revert "compositor: Draw to back buffer"
    
    This change is incompatible with NVidia closed source driver and
    causes a blank screen.
    
    Thanks-to: Philip Müller <philm@manjaro.org>
    
    This reverts commit 97c051b865623cf7133d74d0281ed6ad1cf4c3d6.


commit 97c051b865623cf7133d74d0281ed6ad1cf4c3d6 (refs/bisect/skip-97c051b865623cf7133d74d0281ed6ad1cf4c3d6)
Author: Olivier Fourdan <fourdan@xfce.org>
Date:   Tue May 7 20:25:01 2019 +0200

    compositor: Draw to back buffer
    
    Draw the texture to the back buffer instead of the front buffer and
    `glXSwapBuffers()` will take care of swapping the back to front.
    
    Signed-off-by: Olivier Fourdan <fourdan@xfce.org>

Commits between these two always produce a blank screen on NVidia. By selectively reverting I have found the following:

8a5fc3b with 97c05 reverted: good
f343d4a with 97c05 reverted: blank screen
3eecc5e with f343d4a reverted and then 97c05 reverted: good

So I think the bug is either between 11493fd and 3eecc5e or it is caused by f343d4a.