Branch: refs/heads/main
Home: https://github.com/WebKit/WebKit
Commit: 3056a40d3e2365131be19a444b3f25fe1b42a824
https://github.com/WebKit/WebKit/commit/3056a40d3e2365131be19a444b3f25fe1b42a824
Author: Simon Fraser <[email protected]>
Date: 2024-11-04 (Mon, 04 Nov 2024)
Changed paths:
M
Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingCoordinatorProxy.cpp
M Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingCoordinatorProxy.h
M Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingTree.cpp
M
Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingCoordinatorProxyIOS.h
M Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingTreeIOS.cpp
M Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingTreeIOS.h
M Source/WebKit/UIProcess/RemoteLayerTree/mac/RemoteScrollingTreeMac.mm
Log Message:
-----------
WebCore's main thread scroll position is poorly synchronized with the
UI-process scroll position
https://bugs.webkit.org/show_bug.cgi?id=282215
rdar://138804403
Reviewed by Matt Woodrow.
When we adopted UI-side compositing on macOS, the synchronization between the
UI-process scroll
position, and the web process scroll position became worse. This has
consequences for things like
repainting non-root `background-attachment: fixed`, and for the scroll position
exposed to JavaScript.
It's bad enough that even for a responsive web process (<16ms frames), the web
content scroll position
is often a frame stale for a given rendering update.
There are some race conditions that cause this. A rendering update is triggered
in the UI
process by a displayLink callback off the main thread. This callback triggers
two things:
1. It is bounced to the scrolling thread where it triggers the generation of
synthetic momentum events,
which are handled by the scrolling tree. This results in layer
repositioning, the actual manifestation of scrolling,
and in "scrollingTreeNodeDidScroll" messages, which bounce back to the main
thread, and then result
in IPC to web content. It's these messages that determine the web content
notion of scroll position.
2. It triggers a `DisplayDidRefresh` IPC message from the UI process main
thread to the web process. This
bounces through a zero-delay time in the web process which then starts the
rendering update.
The raciness is between these two series of steps:
i) Display link thread -> scrolling thread -> main thread -> IPC to web
content (for scroll position)
ii) Display link thread -> main thread -> IPC to web content -> zero delay
timer (for rendering update trigger)
`RemoteLayerTreeDrawingAreaProxy::didRefreshDisplay()` tries to address this by
eagerly calling
`scrollingCoordinatorProxy()->sendScrollingTreeNodeDidScroll()` but this was
ineffective because the "node did scroll"
message often hasn't got back to the main thread yet.
The fix is to do what we do already in ThreadedScrollingTree (older the web
process scrolling code): instead of
using a callback to get the "node did scroll" message back to the UI process
main thread, add a `ScrollUpdate`
to the ScrollingTree's pending updates directly from the scrolling thread (with
a lock). With that,
`RemoteLayerTreeDrawingAreaProxy::didRefreshDisplay()` sees the update in time.
`RemoteScrollingCoordinatorProxy::scrollingTreeNodeDidScroll()` goes away,
replaced with `scrollingThreadAddedPendingUpdate`,
and the iOS-specific behavior there about `propagatesMainFrameScrolls` moves
into a `RemoteScrollingTreeIOS::scrollingTreeNodeDidScroll()`
override.
* Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingCoordinatorProxy.cpp:
(WebKit::RemoteScrollingCoordinatorProxy::sendScrollingTreeNodeDidScroll):
(WebKit::RemoteScrollingCoordinatorProxy::scrollingThreadAddedPendingUpdate):
(WebKit::RemoteScrollingCoordinatorProxy::scrollingTreeNodeDidScroll): Deleted.
* Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingCoordinatorProxy.h:
(WebKit::RemoteScrollingCoordinatorProxy::propagatesMainFrameScrolls const):
Deleted.
* Source/WebKit/UIProcess/RemoteLayerTree/RemoteScrollingTree.cpp:
(WebKit::RemoteScrollingTree::scrollingTreeNodeDidScroll):
*
Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingCoordinatorProxyIOS.h:
* Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingTreeIOS.cpp:
(WebKit::RemoteScrollingTreeIOS::scrollingTreeNodeDidScroll):
* Source/WebKit/UIProcess/RemoteLayerTree/ios/RemoteScrollingTreeIOS.h:
* Source/WebKit/UIProcess/RemoteLayerTree/mac/RemoteScrollingTreeMac.mm:
(WebKit::RemoteScrollingTreeMac::scrollingTreeNodeDidScroll):
Canonical link: https://commits.webkit.org/286095@main
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