1612983 - Do not disable APZ when the selection is on nomal position: static elements while pinch-zooming

Status: RESOLVED FIXED

(Core :: DOM: Selection, defect, P3)

Description

[Ting-Yu Lin [:TYLin] (PST, UTC-7)]

Bug 1526268 designed a mechanism that APZ is disabled when AccessibleCaret is in a position:fixed subtree or when its position is changed. In particular, APZ is disabled whenever mIsCaretPositionChanged is true [1]. (See [2] for a more detailed rational)

mIsCaretPositionChanged is updated in when one of the AccessibleCaret has PositionChangedResult::Changed [3]. However, PositionChangedResult::Changed is true when the position is the same, but the zoom level is changed [4]. That's why Bug 1610960 happens that APZ is disabled when pinch-zooming while the selection is on normal position:static text.

[1] https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/layout/base/AccessibleCaretManager.cpp#372
[2] https://phabricator.services.mozilla.com/D51351?id=184974#inline-312215
[3] https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/layout/base/AccessibleCaretManager.cpp#335-337
[4] https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/layout/base/AccessibleCaret.cpp#265,280

Comment 1

[Ting-Yu Lin [:TYLin] (PST, UTC-7)]

Attachment: Bug 1612983 - Disable APZ only when AccessibleCaret's position is changed.

mIsCaretPositionChanged is used in UpdateShouldDisableApz() to
determine whether to disable APZ during scrolling or pinch-zooming.

Suppose the selection is on position:static elements. When
pinch-zooming, the zoom level is changed, but the position is not. We
split PositionChangedResult::Changed into two separate states, and set
mIsCaretPositionChanged only when the position is changed, not when
the zoom level is changed.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/c6dde25bf5fd
Disable APZ only when AccessibleCaret's position is changed. r=botond

Comment 3

[Noemi Erli[:noemi_erli]]

https://hg.mozilla.org/mozilla-central/rev/c6dde25bf5fd