Hi!
When culling in the lighttable (x) and selecting the photo on the right side, then pressing Ctrl+w to display focused areas on this photo, these are the focused areas in the left photo that are displayed.
I don’t think it’s what one would expect, but I may be wrong…
Or is it a bug?
This is a global toggle which acts on every displayed photo, so I’d say that’s the expected behavior. If it were a per-photo setting, it would probably have an icon on each photo like the grouping icon.
Alright, but then why when two photos are displayed only one and always the left one is chosen for focus detection, even though the right is selected?
Often, when you have a series of photos and you’re trying to choose one by comparing them one by one, the first thing you do is check the focus.
So, as it is for now, this would be to be done before comparing photos…
Oh, you’re saying it only renders on the left photo? Is that behavior the same after restarting the program? I’ve noticed focus peaking is not the most stable part of darktable.
You’re right, works randomly. 
Focus detection is a neat feature, but it has befuddled me. I’m not at my editing computer, so I’m posting from memory.
Someone who was a fairly frequent youtube poster with the 2.x series had a video that suggested CTRL+w was sticky. (Stefano Ferro?) It’s never been for me.
In lighttable mode, CTRL+w shows the photo full screen with the focus points. When I release the keys, it reverts to lighttable mode.
I’d LOVE for focus points to be sticky in full screen mode. It would be very helpful to be able to cycle through photos e.g. with left arrow / right arrow to see where it focused and be able to do ratings in this mode.