I am typing these words above a screenshot, taken in PoP!_OS, stored in memory, and pasted into this window. The screenshot is visible now. Previous experience indicates, however, that it will not be visible in the final post.

Here goes...

The menu-legibility problem appears to have manifested itself only when I was initially editing a post. The menu is perfectly legible in the posted version...

I pasted the screenshot in the edit window and it appeared but it is no longer there now that I've posted. This seems like an issue!