Assigning it to myself to investigate.

The issue is reproducible on ToT, both with smooth animation disabled and enabled. 

Still debugging to see why GSUs get ignored.

Deleted: trace_scroll_animation_disabled.json.gz 1.6 MB

trace_scroll_animation_disabled.json.gz 1.6 MB Download

Deleted: trace_trace_with_scroll_animation.json.gz 1.7 MB

trace_trace_with_scroll_animation.json.gz 1.7 MB Download

The issue seems related to layers/hittesting:

In cases that drop down list scrolls successfully https://cs.chromium.org/chromium/src/cc/trees/layer_tree_host_impl.cc?gsn=DistributeScrollDelta&l=3360 forces the scrolling to happen on the main thread. But in cases that scrolling does not happen, IsMainThreadScrolling is not true for any of the scroll node's ancestors, so the GSB and GSUs are handled on Impl and the GSUs get ignored.

Another thing worth mentioning is that, I could not reproduce the bug when dev tools was open.

flackr@ could you please help with finding the proper owner?

Hi, I'm the dev who originally filed this bug on webcompat. Just wanted to let you know that we're currently disabling transitions on this particular .selectable-list in Chrome on Windows, so you'll need to remove this inline style to replicate the bug.

Also, it only appears to affect this dropdown. There's another one under Edit > Vignette that works fine in Chrome on Windows with the opacity transition intact.

Thanks for looking into this!