Bug 276107 – Do not create scrollbar layers (i.e., scrollbar corner) if the subtree is invisible

• Status: NEW
• Component: Scrolling

[Ahmad Saleem]

Hi Team,

While going through Blink's commit, I came across another failing test case:

Test Case: https://jsfiddle.net/d8v0aocs/

^ Shows 'scrollbar corner' while it shouldn't.

Blink Commit: https://chromium.googlesource.com/chromium/blink/+/5d8a13843d41c418d040d3e1c16d37593c091628

WebKit Source: https://searchfox.org/wubkat/rev/00b3d6a56f5149245b9e1501389ae169846ec240/Source/WebCore/rendering/RenderLayerBacking.cpp#2328

Following fixes it:

// If the subtree is invisible, we don't actually need scrollbar layers.
    bool invisible = m_owningLayer.subtreeIsInvisible();
    needsHorizontalScrollbarLayer &= !invisible;
    needsVerticalScrollbarLayer &= !invisible;
    needsScrollCornerLayer &= !invisible;

where 'subtreeIsInvisible' is following in RenderLayer.cpp / RenderLayer.h:

bool subtreeIsInvisible() const { return !hasVisibleContent() && !hasVisibleDescendant(); }

We can also update all call sites in RenderLayer.cpp where this function is used.

Thanks!

[Ahmad Saleem]

Created attachment 471841 [details]
Pre-patch (Scroll Corner) in WebKit

From screenshot, we get 'scroll corner', while it is not the case in other browsers (Chrome Canary 128 and Firefox Nightly 129).

After patch, we also don't get it (will attach another screenshot).

[Ahmad Saleem]

Created attachment 471842 [details]
Post-patch WebKit (no Scroll corner)

[Radar WebKit Bug Importer]

<rdar://problem/131351554>

[Ahmad Saleem]

PR (Closed) - https://github.com/WebKit/WebKit/pull/30569