Shift scroll causes horizontal scroll so it inverts X & Y before the events are delivered.

That definitely seems odd; not sure if anything would break. It appears that events during fling (on chromeos) don't get swapped, but events when the fingers are do do.

rbyers@ Does it seem reasonable to always report deltaY regardless of Shift for a mouse wheel that is in the y direction.

Can't try on Edge; seems they don't fire on a touchpad.. https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/7134034/

Yes, fixing this so that vertical is always deltaY makes sense - especially if some other browsers are doing that (it's what I'd expect).  That behavior is probably another legacy symptom of driving scroll from wheel events.

And FWIW I consider this a bug (not something that should be reflected in any docs) and as such not subject to the blink intent process for breaking changes (unless we uncover non-trivial breakage in real-world pages).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79

commit d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79
Author: dtapuska <dtapuska@chromium.org>
Date: Wed Sep 21 03:39:15 2016

Send the WebMouseWheelEvents unconverted if Shift is down.

WebMouseWheelEvents were converted based on Shift getting set. This should
not happen. Instead swap the X&Y before we goto generate the scroll
gesture.

BUG= 646109 

Review-Url: https://codereview.chromium.org/2359463002
Cr-Commit-Position: refs/heads/master@{#419966}

[modify] https://crrev.com/d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
[modify] https://crrev.com/d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79/ui/events/blink/web_input_event.cc
[modify] https://crrev.com/d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79/ui/events/blink/web_input_event_builders_win.cc
[modify] https://crrev.com/d5d0c8bf89ad9b6a03122b6620bdbbdcd406bb79/ui/events/blink/web_input_event_unittest.cc