This is related to  issue 601584 .

I am disabling external/wpt/html/user-activation/message-event-activation-api-iframe-cross-origin.sub.tentative.html in a mouse capture patch (https://chromium-review.googlesource.com/c/chromium/src/+/1271588) because that change isn't compatible with how cross-process event routing works in layout tests.

In particular, there isn't a compositor when layout tests run so the browser process does not have enough data to perform hit tests to the correct RenderWidgetHostView. Tests were made to pass by causing all mouse input to defer to slow path hit testing in the absence of compositing data.

However, this doesn't fully work when mouse capture is active (or any other mechanism that overrides hit testing as the arbitrator for event targeting), because in that case we have an immediate target but without compositing data we don't have a way to convert the mouse event coordinates from the parent to the child frame.

message-event-activation-api-iframe-cross-origin.sub.tentative.html fails with --site-per-process because the MouseDown causes mouse capture to become active, then the MouseUp does not use hit testing and so does not get converted coordinates from the renderer, and is subsequently dispatched with the wrong coordinates.