An odd effect. Create an album with just a few images in it. On the first image, click on the image to move to the second image. Now mouse on and off the image two or more times. The controls both at upper left and upper right disappear, and reappear only if you mouse over them. And even then, they don't remain visible.
I'm seeing it both in Win10 and macOS, but only in Firefox. Doesn't happen in Chrome.
(I should have remembered that Firefox handles mouseover events in a way that's, well, perverse, for lack of a better term. It's my only consistent complaint about Firefox - otherwise, it's a better browser than Chrome.)