1128740 - Nested scrollbars can have the wrong async transform

Status: RESOLVED FIXED

(Core :: Panning and Zooming, defect)

Description

[David Anderson [:dvander] - inactive, e-mail if emergency]

Attachment: textarea-scrolling.gif

Screen capture of the problem. It looks like when we scroll, the scrollbar of the textarea moves with the outer scrollport's scrollbar, rather than the textarea. It only appears to happen after the textarea becomes scrolled.

The inner scrollbar winds up in the right place but it looks very bad.

Comment 1

[David Anderson [:dvander] - inactive, e-mail if emergency]

Attachment: fix

When scrolling an outer frame, inner scrollbars get the outer async transform applied, but because they're scrollbars then proceed to have that transform clobbered by their local async transform (which is usually nothing). The fix is to just make sure the existing shadow transform is included.

Comment 2

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

Note that this bug would only happen if a layer is both async scrollable and a scrollbar (for an ancestor scrollable layer), which is what was happening in the case dvander posted to IRC:

ContainerLayerComposite (0x7fd184a9a000) [shadow-clip=(x=0, y=0, w=881, h=329)] [shadow-visible=< (x=515, y=146, w=13, h=33); >] [clip=(x=0, y=0, w=881, h=329)] [visible=< (x=515, y=146, w=13, h=33); >] [vscrollbar=4] [metrics0={ [cb=(x=0.000000, y=0.000000, w=881.000000, h=329.000000)] [sr=(x=0.000000, y=0.000000, w=881.000000, h=628.000000)] [s=(0,1)] [dp=(x=0.000000, y=-1.000000, w=881.000000, h=628.000000)] [cdp=(x=0.000000, y=0.000000, w=0.000000, h=0.000000)] [color=rgba(255, 255, 255, 1)] [scrollId=3] [z=1] }]

Comment 3

[Botond Ballo [:botond]]

Comment on attachment 8588722 [details] [diff] [review]
fix

Review of attachment 8588722 [details] [diff] [review]:
-----------------------------------------------------------------

::: gfx/layers/composite/AsyncCompositionManager.cpp
@@ +798,5 @@
>      scrollbarTransform.PostScale(xScale, 1.f, 1.f);
>      scrollbarTransform.PostTranslate(xTranslation, 0, 0);
>    }
>  
> +  Matrix4x4 transform = scrollbarTransform * aScrollbar->GetLocalTransform();

There's a pre-existing bug in this code where it multiplies the async transform (scrollbarTransform) on the left, even though the async transform should apply on top of the regular transform (i.e., should be multiplied on the right).

Let's fix that as well (perhaps in a separate commit for bisection purposes).

Comment 4

[Botond Ballo [:botond]]

(In reply to Kartikaya Gupta (email:[e-mail]) from comment #2)
> Note that this bug would only happen if a layer is both async scrollable and
> a scrollbar (for an ancestor scrollable layer), which is what was happening
> in the case dvander posted to IRC:

Heh, multi-layer-apz gives rise to all sorts of fun combinations :)

Comment 5

[Botond Ballo [:botond]]

I plan to write some tests in bug 1151617 that should make touching this code a little less scary than it is right now.

Comment 6

[David Anderson [:dvander] - inactive, e-mail if emergency]

remote:   https://hg.mozilla.org/integration/mozilla-inbound/rev/86a231570055
remote:   https://hg.mozilla.org/integration/mozilla-inbound/rev/f8d74ba6b0de

Comment 7

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/86a231570055
https://hg.mozilla.org/mozilla-central/rev/f8d74ba6b0de