Markus Stange [:mstange] Reporter
	Description • 10 years ago

With the patches from bug 944938 applied, and the system scrollbar visibility pref set to "always visible", strange things happen.

For example:
 - In the resting position, the purple layer border does not separate the scrollbars from the page (as it should), but instead reaches halfway into the scrollbars. This may be related to HiDPI.
 - When both horizontal and vertical scrollbars are visible, parts of the scrolled page sometimes show through the scroll corner. Instead, the page should be clipped to the inner rect that is framed by the scrollbars.
 - Also with both scrollbars visible, when scrolled to the very end, the scrollbars themselves are clipped away by the purple layer border.

	Markus Stange [:mstange] Reporter
	Comment 2 • 10 years ago

(In reply to Markus Stange [:mstange] from comment #0)
> With the patches from bug 944938 applied, and the system scrollbar
> visibility pref set to "always visible", strange things happen.
> 
> For example:
>  - In the resting position, the purple layer border does not separate the
> scrollbars from the page (as it should), but instead reaches halfway into
> the scrollbars. This may be related to HiDPI.

It is, and there's this helpful comment in nsLayoutUtils.cpp near one of the callers of GetActualScrollbarSizes() (but both callers are affected):
// Scrollbars are not subject to scaling, so CSS pixels = layer pixels for them.

>  - When both horizontal and vertical scrollbars are visible, parts of the
> scrolled page sometimes show through the scroll corner. Instead, the page
> should be clipped to the inner rect that is framed by the scrollbars.
>  - Also with both scrollbars visible, when scrolled to the very end, the
> scrollbars themselves are clipped away by the purple layer border.

In order to fix this, I think we need to get rid of the distinction between display ports on root sub documents and display ports on subframes. The APZ-scrollable ContainerLayer really needs to have composition bounds that only cover the area without scrollbars. If the scrollbars themselves are inside that container layer, they'll be clipped away. So what we want is a layer tree like this:
- ContainerLayer for nsDisplaySubdocument that covers viewport + scrollbars
  - ContainerLayers for scrollbars (nsDisplayOwnLayer) (either under the scrollable layer or on top, depending on whether they're overlay scrollbars)
  - ContainerLayer for scroll frame (nsDisplayScrollLayer)
    - layers for page content

	Markus Stange [:mstange] Reporter
	Comment 3 • 9 years ago

This seems mostly fixed now.