Photos.app Edit-mode UI broken after upgrading to High Sierra
Earlier today, I discovered that if I clicked where the pull-down arrow was supposed to be, the application behaved as expected, and the additional options dropped down. 

Some kind of bug, I thought.

It turns out, I had "Increase Contrast" turned on in Accessibility system preferences.  When I toggle this on, the triangle there, but only +2% lighter than the surrounding pixels:  #202020 on an #1C1C1C background, a +2% difference, as reported by Digital Color Meter.app.  When I uncheck that option, the triangles appear, as #3C3C3C  on an #1C1C1C background, a +9% difference.