Steps to replicate the issue (include links if applicable):

• Browse a wiki using a mobile device and choose desktop view
• Go to for example https://en.wikipedia.org/wiki/Special:Log
• Press the "All public logs" dropdown menu so that all log options are dropped down
• Zoom in on all the logs or just scroll down the page

What happens?:

On enwiki:

Zooming in or scrolling down, at first all text disappears but the borders of the dropdown window are still visable. So it's basically a big white square. Zooming in or scrolling down even more, the entire window with its borders is removed and you can now display what is behind it.

On test2wiki:

The first stage - the text being removed and just a big white square is displayed - didn't reproduce here. It looked find until this point. Zooming in or scrolling down even more however and it was the same problem as on enwiki, that is, the entire dropdown menu disappeared and you could see what's behind it.

What should have happened instead?:
Nothing except that the page is zoomed or scrolled down.

Software version (skip for WMF-hosted wikis like Wikipedia):

This was found using a mobile device with desktop view. Both the Safari and Chrome apps gave the same results. iOS on iPhone SE (2nd generation). Using a PC I couldn't reproduce this.