Tiled layer flicker when an animation ends

https://bugs.webkit.org/show_bug.cgi?id=244017
<rdar://89111231>

Reviewed by Tim Horton.

The test page (internal to Apple) had a tiled layer whose transform was animated via a transition.
When the animation finished, there was sometimes a frame where tiles were missing, causing a visual
flash. When tiled layers are being animated, we wake up every frame to re-evaluate the tile
coverage.

Transform animations are implemented via Core Animation; the code makes a fill-forwards animation on
the layer. For such animations, `DocumentTimeline::timeToNextTick()` ensures that we don't waste
power doing rendering updates on every frame, scheduling a timer to fire for the next relevant time.

However, it was possible for this timer to fire late; this resulted in a frame where the tile
coverage logic, which consults DocumentTimeline animation state, saw an animation that had completed
(and computed tile coverage on that basis), but the CAAnimation was still affecting the rendered
result, putting the tiled layer in a position where some areas without tile coverage were exposed.

Fix by having the m_tickScheduleTimer fire slightly (~16ms) earlier, so that DocumentTimeline
animations have their correct state when consulted for tile coverage computation. This may result in
a few more rendering updates at the ends of accelerated animations (as detected by layout tests),
but that should be benign.

* LayoutTests/animations/no-style-recalc-during-accelerated-animation.html:
* LayoutTests/animations/steps-transform-rendering-updates-expected.txt:
* LayoutTests/animations/steps-transform-rendering-updates.html:
* Source/WebCore/animation/DocumentTimeline.cpp:
(WebCore::DocumentTimeline::scheduleAnimationResolution):
(WebCore::DocumentTimeline::scheduleNextTick):

Canonical link: https://commits.webkit.org/253549@main

LayoutTests/animations/no-style-recalc-during-accelerated-animation-expected.txt

@@ -1,2 +1,2 @@
 Got iteration event.
-PASS: saw three or fewer style recalcs during the animation.
+PASS: saw five or fewer style recalcs during the animation.

LayoutTests/animations/no-style-recalc-during-accelerated-animation.html

@@ -35,10 +35,10 @@
             box.addEventListener("animationiteration", () => result.innerText = "Got iteration event.\n");
             box.addEventListener("animationend", () => {
                 const numRecalcs = internals.styleRecalcCount();
-                if (numRecalcs > 3)
-                    result.innerText += "FAIL: saw " + numRecalcs + " style recalcs during the animation, should only see three."
+                if (numRecalcs > 5)
+                    result.innerText += "FAIL: saw " + numRecalcs + " style recalcs during the animation, should only see five."
                 else 
-                    result.innerText += "PASS: saw three or fewer style recalcs during the animation."
+                    result.innerText += "PASS: saw five or fewer style recalcs during the animation."
 
                 if (window.testRunner)
                     testRunner.notifyDone();

LayoutTests/animations/steps-transform-rendering-updates-expected.txt

@@ -1,5 +1,5 @@
 PASS count is 0
-PASS count < 6 is true
+PASS count < 7 is true
 PASS successfullyParsed is true
 
 TEST COMPLETE

LayoutTests/animations/steps-transform-rendering-updates.html

@@ -35,7 +35,7 @@
             }, false);
             box.addEventListener('animationend', () => {
                 count = internals.renderingUpdateCount();
-                shouldBeTrue('count < 6');
+                shouldBeTrue('count < 7');
                 finishJSTest();
             }, false);
 

Source/WebCore/animation/DocumentTimeline.cpp

@@ -167,8 +167,11 @@ void DocumentTimeline::scheduleAnimationResolution()
     if (animationsAreSuspended() || m_animationResolutionScheduled || !m_document || !m_document->page())
         return;
 
+    bool havePendingActivity = shouldRunUpdateAnimationsAndSendEventsIgnoringSuspensionState();
+    LOG_WITH_STREAM(Animations, stream << "DocumentTimeline " << this << " scheduleAnimationResolution - havePendingActivity " << havePendingActivity);
+
     // We need some relevant animations or pending events to proceed.
-    if (!shouldRunUpdateAnimationsAndSendEventsIgnoringSuspensionState())
+    if (!havePendingActivity)
         return;
 
     m_document->page()->scheduleRenderingUpdate(RenderingUpdateStep::Animations);
@@ -342,6 +345,8 @@ void DocumentTimeline::scheduleNextTick()
         return std::nullopt;
     };
 
+    auto animationInterval = this->animationInterval();
+
     for (const auto& animation : m_animations) {
         if (!animation->isRelevant())
             continue;
@@ -364,15 +369,19 @@ void DocumentTimeline::scheduleNextTick()
                 animationTimeToNextRequiredTick = *timeToNextPossibleTickAccountingForFrameRate - nextTickTimeEpsilon;
         }
 
-        if (animationTimeToNextRequiredTick < animationInterval()) {
+        if (animationTimeToNextRequiredTick < animationInterval) {
+            LOG_WITH_STREAM(Animations, stream << "DocumentTimeline " << this << " scheduleNextTick - scheduleAnimationResolution now");
             scheduleAnimationResolution();
             return;
         }
         scheduleDelay = std::min(scheduleDelay, animationTimeToNextRequiredTick);
     }
 
-    if (scheduleDelay < Seconds::infinity())
+    if (scheduleDelay < Seconds::infinity()) {
+        scheduleDelay = std::max(scheduleDelay - animationInterval, 0_s);
+        LOG_WITH_STREAM(Animations, stream << "DocumentTimeline " << this << " scheduleNextTick - scheduleDelay " << scheduleDelay);
         m_tickScheduleTimer.startOneShot(scheduleDelay);
+    }
 }
 
 void DocumentTimeline::animationAcceleratedRunningStateDidChange(WebAnimation& animation)