981651 - PositionedEventTargeting doesn't work so well when zoomed in with APZ

Status: RESOLVED FIXED

(Core :: Layout, defect)

Description

[Kartikaya Gupta (email:[e-mail])]

It looks like the AppUnitsFromMM code in PositionedEventTargeting.cpp takes into account the presshell resolution when converting from MM to app units. However, it doesn't take into account the async zoom and/or the layer transforms which should mostly (or competely) cancel out the resolution that has been set.

The net effect is that if you go to a page and zoom in, tapping on empty space will often select a link that is visually quite far away (although it's relatively close in terms of CSS pixels). This can be quite a frustrating experience at times.

STR:
1. In the B2G browser, load a page like https://people.mozilla.com/~kgupta/grid.html
2. Zoom in a bunch (around 3x is a good amount, so one grid square takes up around a screen width on a 320px device)
3. Try tapping in the empty space in the bottom half of the grid squares

Expected results:
Taps are ignored unless they are actually close to a link

Actual results:
Some link is almost always activated, even if on the screen it is pretty far away.

Comment 1

[Kartikaya Gupta (email:[e-mail])]

I think we would get better results by just skipping the resolution there. There are probably some cases where the resolution isn't cancelled out by layer transforms, but I suspect those are less frequent than cases where it is cancelled out. roc, what do you think?

Comment 2

[Kartikaya Gupta (email:[e-mail])]

For comment 1 (hit submit too early)

Comment 3

[Kartikaya Gupta (email:[e-mail])]

Attachment: Patch

This does what I described above and seems to work locally.

Comment 4

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Comment on attachment 8388704 [details] [diff] [review]
Patch

Review of attachment 8388704 [details] [diff] [review]:
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I agree!

Comment 5

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/01de281bca37

Comment 6

[Ryan VanderMeulen [:RyanVM]]

Follow-up for Werror bustage.
https://hg.mozilla.org/integration/mozilla-inbound/rev/3c4fa37bba16

Comment 7

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/01de281bca37
https://hg.mozilla.org/mozilla-central/rev/3c4fa37bba16