Bug 1645954. Don't use a stale value of mScrollPort during scrollframe reflow while still determining the new value of mScrollPort. r=tnikkel

Also, adjust for scrollbars and resolution correctly.

Differential Revision: https://phabricator.services.mozilla.com/D79778

layout/base/nsLayoutUtils.cpp

@@ -8701,14 +8701,16 @@ nsMargin nsLayoutUtils::ScrollbarAreaToExcludeFromCompositionBoundsFor(
 
 /* static */
 nsSize nsLayoutUtils::CalculateCompositionSizeForFrame(
-    nsIFrame* aFrame, bool aSubtractScrollbars) {
+    nsIFrame* aFrame, bool aSubtractScrollbars,
+    const nsSize* aOverrideScrollPortSize) {
   // If we have a scrollable frame, restrict the composition bounds to its
   // scroll port. The scroll port excludes the frame borders and the scroll
   // bars, which we don't want to be part of the composition bounds.
   nsIScrollableFrame* scrollableFrame = aFrame->GetScrollTargetFrame();
   nsRect rect = scrollableFrame ? scrollableFrame->GetScrollPortRect()
                                 : aFrame->GetRect();
-  nsSize size = rect.Size();
+  nsSize size =
+      aOverrideScrollPortSize ? *aOverrideScrollPortSize : rect.Size();
 
   nsPresContext* presContext = aFrame->PresContext();
   PresShell* presShell = presContext->PresShell();

layout/base/nsLayoutUtils.h

@@ -2706,7 +2706,8 @@ class nsLayoutUtils {
    * are likely to need special-case handling of the RCD-RSF.
    */
   static nsSize CalculateCompositionSizeForFrame(
-      nsIFrame* aFrame, bool aSubtractScrollbars = true);
+      nsIFrame* aFrame, bool aSubtractScrollbars = true,
+      const nsSize* aOverrideScrollPortSize = nullptr);
 
   /**
    * Calculate the composition size for the root scroll frame of the root

layout/generic/nsGfxScrollFrame.cpp

@@ -464,14 +464,16 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
   nsSize visualViewportSize = scrollPortSize;
   mozilla::PresShell* presShell = PresShell();
   if (mHelper.mIsRoot && presShell->IsVisualViewportSizeSet()) {
-    nsSize compositionSize =
-        nsLayoutUtils::CalculateCompositionSizeForFrame(this, false);
+    visualViewportSize = nsLayoutUtils::CalculateCompositionSizeForFrame(
+        this, false, &layoutSize);
+
+    visualViewportSize = nsSize(
+        std::max(0, visualViewportSize.width - vScrollbarDesiredWidth),
+        std::max(0, visualViewportSize.height - hScrollbarDesiredHeight));
+
     float resolution = presShell->GetResolution();
-    compositionSize.width /= resolution;
-    compositionSize.height /= resolution;
-    visualViewportSize =
-        nsSize(std::max(0, compositionSize.width - vScrollbarDesiredWidth),
-               std::max(0, compositionSize.height - hScrollbarDesiredHeight));
+    visualViewportSize.width /= resolution;
+    visualViewportSize.height /= resolution;
   }
 
   nsRect overflowRect = aState->mContentsOverflowAreas.ScrollableOverflow();