Bug 321908 – REGRESSION(309315@main): [WPE][GTK] position:fixed layers are unnecessarily repainted by scrolling

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKitGTK

[Fujii Hironori]

REGRESSION(309315@main): [WPE][GTK] position:fixed layers are unnecessarily repainted by scrolling

1. Start GTK MiniBrowser
2. Open WebInspector
3. Enable show compositing border by clicking the button in Element tab
4. Load the content
5. Scroll the page by mouse wheel

Actual: the repainting counter of the float layer is rapidly increasing while scrolling.
Expected: the repainting counter of the float layer is not increasing while scrolling.

Reverting 309315@main fixed this problem.

[Fujii Hironori]

Created attachment 481072 [details]
test content

[Fujii Hironori]

After 309315@main, in LocalFrameView::updateLayoutViewport, the following condition becomes true. Then, layout is triggered.
https://github.com/WebKit/WebKit/blob/9c27c4ccf517794644bc3e08facc1e95a14f4e75/Source/WebCore/page/LocalFrameView.cpp#L2017

>    if (newLayoutViewportOrigin != m_layoutViewportOrigin) {
>        setBaseLayoutViewportOrigin(newLayoutViewportOrigin);

I added the debug logging.

diff --git a/Source/WebCore/page/LocalFrameView.cpp b/Source/WebCore/page/LocalFrameView.cpp
index 08785a978430..ca2382d69f75 100644
--- a/Source/WebCore/page/LocalFrameView.cpp
+++ b/Source/WebCore/page/LocalFrameView.cpp
@@ -2014,6 +2014,7 @@ void LocalFrameView::updateLayoutViewport()
     }
 
     LayoutPoint newLayoutViewportOrigin = computeLayoutViewportOrigin(visualViewportRect(), minStableLayoutViewportOrigin(), maxStableLayoutViewportOrigin(), layoutViewport, scrollBehaviorForFixedElements());
+    WTF_ALWAYS_LOG(__FUNCTION__ << ":" << __LINE__ << " " << this << " cond:" << (newLayoutViewportOrigin != m_layoutViewportOrigin) << " newLayoutViewportOrigin:" << newLayoutViewportOrigin << " m_layoutViewportOrigin:" << m_layoutViewportOrigin);
     if (newLayoutViewportOrigin != m_layoutViewportOrigin) {
         setBaseLayoutViewportOrigin(newLayoutViewportOrigin);
         LOG_WITH_STREAM(Scrolling, stream << "layoutViewport changed to " << layoutViewportRect());

Then, I got the following log:

> updateLayoutViewport:2017 0x7fd6f8000000 cond:0 newLayoutViewportOrigin:(0.00,274.00) m_layoutViewportOrigin:(0.00,274.00)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:1 newLayoutViewportOrigin:(0.00,293.00) m_layoutViewportOrigin:(0.00,292.97)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:1 newLayoutViewportOrigin:(0.00,293.00) m_layoutViewportOrigin:(0.00,292.97)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:0 newLayoutViewportOrigin:(0.00,293.00) m_layoutViewportOrigin:(0.00,293.00)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:1 newLayoutViewportOrigin:(0.00,322.00) m_layoutViewportOrigin:(0.00,321.61)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:1 newLayoutViewportOrigin:(0.00,322.00) m_layoutViewportOrigin:(0.00,321.61)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:0 newLayoutViewportOrigin:(0.00,322.00) m_layoutViewportOrigin:(0.00,322.00)
> updateLayoutViewport:2017 0x7fd6f8000000 cond:1 newLayoutViewportOrigin:(0.00,349.00) m_layoutViewportOrigin:(0.00,348.70)

The fractional parts are different.

m_layoutViewportOrigin of LocalFrameView is LayoutPoint type.
newLayoutViewportOrigin comes from m_scrollPosition of ScrollView which is IntPoint type.

[Fujii Hironori]

There are several ways to work around. This is one of them.

diff --git a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
index 257b00d4f287..5739400272af 100644
--- a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
+++ b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
@@ -873,6 +873,9 @@ void AsyncScrollingCoordinator::reconcileScrollingState(LocalFrameView& frameVie
     if (layoutViewportOriginOrOverrideRect) {
         WTF::switchOn(*layoutViewportOriginOrOverrideRect,
             [&frameView](FloatPoint origin) {
+#if USE(COORDINATED_GRAPHICS)
+                origin = roundedIntPoint(origin);
+#endif
                 frameView.setBaseLayoutViewportOrigin(LayoutPoint(origin), LocalFrameView::TriggerLayoutOrNot::No);
             }, [&layoutViewportRect](FloatRect overrideRect) {
                 layoutViewportRect = overrideRect;

[Fujii Hironori]

Pull request: https://github.com/WebKit/WebKit/pull/71996

[EWS]

Committed 320147@main (13cc83ffe56b): <https://commits.webkit.org/320147@main>

Reviewed commits have been landed. Closing PR #71996 and removing active labels.