Intent to change: Enable synthetic mousemove on Android to dispatch mouse/pointer boundary events at a layout change or a scroll
Masayuki Nakano
Summary: We've not dispatched mouse/pointer boundary events at the last mouse cursor position when a layout change or a scroll occurs on Android. So, I'd like to enable this behavior on Android for conforming to current Pointer Events.
Details: We're dispatching an internal synthetic `mousemove`
when a layout change or a scroll occurs but not exposed to the
web. Then, it triggers the default handlers of `mousemove`,
so, mouse boundary events and pointer boundary events may be fired
if and only if the element at the last mouse cursor position is
changed. And also the `:hover` state is changed to
the element. However, this has been disabled on Android because
after swiping the page to scroll, the synthetic `mousemove`
will be fired at originally touched point and an element which is
moved at the point will get `:hover` state. So, in
the touch input case, keeping storing the last touch point is odd
in some cases.
I realized that when APZC starts handling a gesture of a touch,
APZC synthesizes a `pointercancel`. So, if `PresShell`
starts handling `ePointerCanel` or `ePointUp`
caused by touch input device, `PresShell` should
forget the last mouse cursor position if and only if the position
is caused by another touch input. Then, synthetic `mousemove`
won't be fired after the touch is released or canceled by a
gesture.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1633450
Specification: https://w3c.github.io/pointerevents/#boundary-events-caused-by-layout-changes
Platform Coverage: All (We've already enabled in desktop, but now enabled on Android too)
Preference: layout.reflow.synthMouseMove
Web-platform tests: https://wpt.fyi/results/pointerevents/pointerevent_pointerout_no_pointer_movement.html?label=master&label=experimental&aligned
NOTE: This is a risky change but we should fix this before mobile OSes would be used in wpt.fyi. So, please file regressions if you find.
-- Masayuki Nakano <masa...@d-toybox.com> Working on DOM, Events, editor and IME handling for Gecko