Bug 1634616 - Unscale pre-render area for transform animations too. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D74520

layout/base/nsLayoutUtils.cpp

@@ -9810,13 +9810,19 @@ CSSPoint nsLayoutUtils::GetCumulativeApzCallbackTransform(nsIFrame* aFrame) {
 
 /* static */
 nsRect nsLayoutUtils::ComputePartialPrerenderArea(
-    const nsRect& aDirtyRect, const nsRect& aOverflow,
+    nsIFrame* aFrame, const nsRect& aDirtyRect, const nsRect& aOverflow,
     const nsSize& aPrerenderSize) {
+  const gfxSize scale = GetTransformToAncestorScale(aFrame);
+  nsSize prerenderSize = aPrerenderSize;
+  if (scale.width != 0 && scale.height != 0) {
+    prerenderSize.width /= scale.width;
+    prerenderSize.height /= scale.height;
+  }
   // Simple calculation for now: center the pre-render area on the dirty rect,
   // and clamp to the overflow area. Later we can do more advanced things like
   // redistributing from one axis to another, or from one side to another.
-  nscoord xExcess = std::max(aPrerenderSize.width - aDirtyRect.width, 0);
-  nscoord yExcess = std::max(aPrerenderSize.height - aDirtyRect.height, 0);
+  nscoord xExcess = std::max(prerenderSize.width - aDirtyRect.width, 0);
+  nscoord yExcess = std::max(prerenderSize.height - aDirtyRect.height, 0);
   nsRect result = aDirtyRect;
   result.Inflate(xExcess / 2, yExcess / 2);
   return result.MoveInsideAndClamp(aOverflow);

layout/base/nsLayoutUtils.h

@@ -2970,16 +2970,21 @@ class nsLayoutUtils {
   /**
    * Compute a rect to pre-render in cases where we want to render more of
    * something than what is visible (usually to support async transformation).
-   * @param aDirtyRect the area that's visible
-   * @param aOverflow the total size of the thing we're rendering
-   * @param aPrerenderSize how large of an area we're willing to render
+   * @param aFrame the target frame to be pre-rendered
+   * @param aDirtyRect the area that's visible in the coordinate system of
+   *        |aFrame|.
+   * @param aOverflow the total size of the thing we're rendering in the
+   *        coordinate system of |aFrame|.
+   * @param aPrerenderSize how large of an area we're willing to render in the
+   *        coordinate system of the root frame.
    * @return A rectangle that includes |aDirtyRect|, is clamped to |aOverflow|,
    *         and is no larger than |aPrerenderSize| (unless |aPrerenderSize|
    *         is smaller than |aDirtyRect|, in which case the returned rect
    *         will still include |aDirtyRect| and thus be larger than
    *         |aPrerenderSize|).
    */
-  static nsRect ComputePartialPrerenderArea(const nsRect& aDirtyRect,
+  static nsRect ComputePartialPrerenderArea(nsIFrame* aFrame,
+                                            const nsRect& aDirtyRect,
                                             const nsRect& aOverflow,
                                             const nsSize& aPrerenderSize);
 

layout/painting/nsDisplayList.cpp

@@ -7827,8 +7827,8 @@ auto nsDisplayTransform::ShouldPrerenderTransformedContent(
   }
 
   if (StaticPrefs::layout_animation_prerender_partial()) {
-    *aDirtyRect = nsLayoutUtils::ComputePartialPrerenderArea(*aDirtyRect,
-                                                             overflow, maxSize);
+    *aDirtyRect = nsLayoutUtils::ComputePartialPrerenderArea(
+        aFrame, *aDirtyRect, overflow, maxSize);
     result.mDecision = PrerenderDecision::Partial;
     return result;
   }

layout/xul/nsSliderFrame.cpp

@@ -313,14 +313,9 @@ void nsSliderFrame::BuildDisplayListForChildren(
 
       const nsRect overflow = thumb->GetVisualOverflowRectRelativeToParent();
       nsSize refSize = aBuilder->RootReferenceFrame()->GetSize();
-      const gfxSize scale = nsLayoutUtils::GetTransformToAncestorScale(thumb);
-      if (scale.width != 0 && scale.height != 0) {
-        refSize.width /= scale.width;
-        refSize.height /= scale.height;
-      }
       nsRect dirty = aBuilder->GetVisibleRect().Intersect(thumbRect);
       dirty = nsLayoutUtils::ComputePartialPrerenderArea(
-          aBuilder->GetVisibleRect(), overflow, refSize);
+          thumb, aBuilder->GetVisibleRect(), overflow, refSize);
 
       nsDisplayListBuilder::AutoBuildingDisplayList buildingDisplayList(
           aBuilder, this, dirty, dirty);