Bug 2065180 - Deal with abspos ASRs correctly when building the top layer. r=tnikkel,layout-reviewers,layout-anchor-positioning-reviewers,dshin

Cleaned-up version of D324081.

The assertion in the bug is one symptom; the other is that such a frame
doesn't follow its anchor during an async scroll, which the reftest
covers.

Co-Authored-By: Dão Gottwald <dao@mozilla.com>

Differential Revision: https://phabricator.services.mozilla.com/D324111

layout/base/DisplayPortUtils.cpp

@@ -1289,6 +1289,40 @@ const ActiveScrolledRoot* DisplayPortUtils::ActivateDisplayportOnASRAncestors(
   return asr;
 }
 
+const ActiveScrolledRoot* DisplayPortUtils::GetASRForAbsPosFrame(
+    nsIFrame* aFrame, const ActiveScrolledRoot* aContainingBlockASR,
+    nsDisplayListBuilder* aBuilder) {
+  MOZ_ASSERT(aFrame->IsAbsolutelyPositioned());
+  if (!aBuilder->IsPaintingToWindow() ||
+      // If we are in view transition capture we get a null asr no matter
+      // what, so don't bother checking for async scrolling with a CSS anchor
+      // pos anchor.
+      aBuilder->IsInViewTransitionCapture() ||
+      // If there is an active view transition in this document it is tricky
+      // to determine what will be an active scroll frame outside of that
+      // frame's BuildDisplayList, so don't bother to async scroll with an
+      // anchor in that case. Bug 2001861 tracks removing this check.
+      aFrame->PresContext()->Document()->GetActiveViewTransition()) {
+    return aContainingBlockASR;
+  }
+  nsIFrame* scrollsWithAnchor =
+      AnchorPositioningUtils::GetAnchorThatFrameScrollsWith(aFrame, aBuilder);
+  if (!scrollsWithAnchor) {
+    return aContainingBlockASR;
+  }
+  if (aBuilder->IsRetainingDisplayList()) {
+    if (aBuilder->IsPartialUpdate()) {
+      aBuilder->SetPartialBuildFailed(true);
+    } else {
+      aBuilder->SetDisablePartialUpdates(true);
+    }
+  }
+  // TODO should we set the scroll parent id too?
+  // https://github.com/w3c/csswg-drafts/issues/12042
+  return ActivateDisplayportOnASRAncestors(
+      scrollsWithAnchor, aFrame->GetParent(), aContainingBlockASR, aBuilder);
+}
+
 static bool CheckAxes(ScrollContainerFrame* aScrollFrame, PhysicalAxes aAxes) {
   if (aAxes == kPhysicalAxesBoth) {
     return true;

layout/base/DisplayPortUtils.h

@@ -406,6 +406,13 @@ class DisplayPortUtils {
       const ActiveScrolledRoot* aASRofLimitAncestor,
       nsDisplayListBuilder* aBuilder);
 
+  /**
+   * Returns the ASR that an abspos frame `aFrame` should scroll with.
+   */
+  static const ActiveScrolledRoot* GetASRForAbsPosFrame(
+      nsIFrame* aFrame, const ActiveScrolledRoot* aContainingBlockASR,
+      nsDisplayListBuilder* aBuilder);
+
   /**
    * aFrame is an absolutely positioned frame that is anchor positioned and
    * compensates for scroll in at least one axis.

layout/generic/ViewportFrame.cpp

@@ -12,6 +12,7 @@
 #include "MobileViewportManager.h"
 #include "mozilla/AbsoluteContainingBlock.h"
 #include "mozilla/ComputedStyleInlines.h"
+#include "mozilla/DisplayPortUtils.h"
 #include "mozilla/PresShell.h"
 #include "mozilla/ProfilerLabels.h"
 #include "mozilla/ReflowInput.h"
@@ -133,8 +134,11 @@ static void BuildDisplayListForTopLayerFrame(nsDisplayListBuilder* aBuilder,
       // root scroll frame.
       clipState.SetClipChainForContainingBlockDescendants(
           savedOutOfFlowData->mCombinedClipChain);
+
       asrSetter.SetCurrentActiveScrolledRoot(
-          savedOutOfFlowData->mContainingBlockActiveScrolledRoot);
+          DisplayPortUtils::GetASRForAbsPosFrame(
+              aFrame, savedOutOfFlowData->mContainingBlockActiveScrolledRoot,
+              aBuilder));
       asrSetter.SetCurrentScrollParentId(savedOutOfFlowData->mScrollParentId);
     }
   }

layout/generic/nsIFrame.cpp

@@ -4607,30 +4607,6 @@ void nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder* aBuilder,
   if (savedOutOfFlowData) {
     aBuilder->SetBuildingInvisibleItems(false);
 
-    nsIFrame* scrollsWithAnchor = nullptr;
-    if (aBuilder->IsPaintingToWindow() &&
-        // If we are in view transition capture we get a null asr no matter
-        // what, so don't bother checking for async scrolling with a CSS anchor
-        // pos anchor.
-        !aBuilder->IsInViewTransitionCapture() &&
-        child->IsAbsolutelyPositioned(disp) &&
-        // If there is an active view transition in this document it is tricky
-        // to determine what will be an active scroll frame outside of that
-        // frame's BuildDisplayList, so don't bother to async scroll with an
-        // anchor in that case. Bug 2001861 tracks removing this check.
-        !PresContext()->Document()->GetActiveViewTransition()) {
-      scrollsWithAnchor = AnchorPositioningUtils::GetAnchorThatFrameScrollsWith(
-          child, aBuilder);
-
-      if (scrollsWithAnchor && aBuilder->IsRetainingDisplayList()) {
-        if (aBuilder->IsPartialUpdate()) {
-          aBuilder->SetPartialBuildFailed(true);
-        } else {
-          aBuilder->SetDisablePartialUpdates(true);
-        }
-      }
-    }
-
     const ActiveScrolledRoot* asr =
         savedOutOfFlowData->mContainingBlockActiveScrolledRoot;
 
@@ -4669,12 +4645,8 @@ void nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder* aBuilder,
     }
 #endif
 
-    if (scrollsWithAnchor) {
-      asr = DisplayPortUtils::ActivateDisplayportOnASRAncestors(
-          scrollsWithAnchor, child->GetParent(), asr, aBuilder);
-
-      // TODO should we set the scroll parent id too?
-      // https://github.com/w3c/csswg-drafts/issues/12042
+    if (child->IsAbsolutelyPositioned(disp)) {
+      asr = DisplayPortUtils::GetASRForAbsPosFrame(child, asr, aBuilder);
     }
 
     if (aBuilder->IsInViewTransitionCapture()) {

layout/reftests/async-scrolling/anchor-pos-top-layer-1-ref.html

@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html>
+<style>
+.anchor {
+  position: absolute;
+  left: 0;
+  top: 50px;
+  width: 50px;
+  height: 50px;
+  background: blue;
+}
+.anchored {
+  position: absolute;
+  left: 50px;
+  top: 50px;
+  width: 50px;
+  height: 50px;
+  background: yellow;
+}
+.child {
+  position: absolute;
+  left: 50px;
+  top: 50px;
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+</style>
+<div class="anchor"></div>
+<div class="anchored"></div>
+<div class="child"></div>
+</html>

layout/reftests/async-scrolling/anchor-pos-top-layer-1.html

@@ -0,0 +1,62 @@
+<!DOCTYPE html>
+<html reftest-async-scroll>
+<style>
+.scroller {
+  position: absolute;
+  left: 0;
+  top: 0;
+  overflow: auto;
+  scrollbar-width: none;
+  width: 150px;
+  height: 100px;
+}
+.spacer {
+  width: 1px;
+  height: 100px;
+}
+.largespacer {
+  width: 1px;
+  height: 500px;
+}
+.anchor {
+  width: 50px;
+  height: 50px;
+  background: blue;
+  anchor-name: --my-anchor;
+}
+#popover {
+  margin: 0;
+  border: none;
+  padding: 0;
+  overflow: visible;
+  position: absolute;
+  position-anchor: --my-anchor;
+  position-visibility: always;
+  left: anchor(right);
+  top: anchor(top);
+  width: 50px;
+  height: 50px;
+  background: yellow;
+}
+#popover > div {
+  position: absolute;
+  left: 0;
+  top: 0;
+  width: 10px;
+  height: 10px;
+  background: green;
+}
+</style>
+<div class="scroller"
+  reftest-displayport-x="0" reftest-displayport-y="0"
+  reftest-displayport-w="150" reftest-displayport-h="500"
+  reftest-async-scroll-y="50">
+  <div class="spacer"></div>
+  <div class="anchor"></div>
+  <div class="largespacer"></div>
+</div>
+<div id="popover" popover><div></div></div>
+<script>
+  document.getElementById("popover").showPopover();
+</script>
+</html>

layout/reftests/async-scrolling/reftest.list

@@ -202,3 +202,4 @@ defaults
 == anchor-pos-sticky-3.html anchor-pos-sticky-3-ref.html
 == anchor-pos-sticky-4.html anchor-pos-sticky-4-ref.html
 == anchor-pos-one-axis-1.html anchor-pos-one-axis-1-ref.html
+== anchor-pos-top-layer-1.html anchor-pos-top-layer-1-ref.html