1638806 - Additional mouseenter are fired while switching to another TAB via keyboard shortcut

Status: RESOLVED FIXED

(Core :: DOM: UI Events & Focus Handling, defect, P2)

Description

[Edgar Chen [:edgar] (OOO)]

Found this bug while investigating bug 1638091 and bug 1635784.

STR

1. Open multiple tabs.
2. Load https://codepen.io/edgarchen-the-decoder/pen/ExVONrm in one of the tabs.
3. Move the mouse to the green area.
4. Switch to another tab via keyboard shortcut (control+tab in MAC for example)
5. Switch to the condepen tab via mouse click.

Expected result

Only mouseenter and mouseleave are logged in the green area.
The mouseenter is from step 3.
The mouseleave is fired because of switching to another tab in step 4.

Actual result

Additional mouseenter is fired.

Comment 1

[Edgar Chen [:edgar] (OOO)]

Attachment: Bug 1638806 - Do not update sLastMouseRemoteTarget for eMouseExitFromWidget event;

Comment 2

[Edgar Chen [:edgar] (OOO)]

This is actually a regression of bug 1607375.

What happens is while we switch from one tab to another by a keyboard shortcut, the synthesized mouse move event now dispatch to the new tab. And because of mLastOverElement still points to the old tab, so we will:

1. Generate and dispatch a eMouseExitFromWidget to the old tab.
2. Generate and dispatch a eMouseEnterIntoWidget to the new tab. At this step, since the sLastMouseRemoteTarget still pointers to the old tab, so we generate an additional eMouseExitFromWidget to the old tab in https://searchfox.org/mozilla-central/rev/9193635dca8cfdcb68f114306194ffc860456044/dom/events/EventStateManager.cpp#1324-1327.

Comment 3

[Edgar Chen [:edgar] (OOO)]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=eee5e9aa48e400d3d9aae58c0023c43c60dc52dd

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

Set release status flags based on info from the regressing bug 1607375

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/56423aee609e
Do not update sLastMouseRemoteTarget for eMouseExitFromWidget event; r=smaug

Comment 6

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/56423aee609e