Photos.app Edit-mode UI broken after upgrading to High Sierra Since upgrading to macos High Sierra, my Photos.app's editing functions have been troublesome. Features such as Retouch simply do not work, and I'm unable to access advanced options, such as the Shadows slider commonly found under the "Light" adjustment.

OSXDaily put out a troubleshooting guide for HS apps today which included a "Create a new account to test app" suggestion, which I did.  On the new test account, Photos.app's Edit features work as they're supposed to; Having not seen it work properly before, I did not know what to expect, nor what was missing prior.

On my original login, Photos.app's Edit screen doesn't show the pull-down arrows that should be to the left of Options.  Without that pull-down, most features are impossible to access.  I've tried re-sizing the window's internal and external dimensions, changing screen resolutions, and changing the scroll-bar behaviour.

I blew away the ~/Library/Preferences/Photos.plist files, to no avail. I'm not sure what other configuration files could be interfering with the Photos.app editing UI.

I'm not sure how to proceed without clobbering my current login account.
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.