Bug 1359868 - Move the ComputePartialPrerenderArea() helper to nsLayoutUtils so it can be reused. r=mstange

MozReview-Commit-ID: GVRBUfYwOFP

--HG--
extra : rebase_source : 8e16daf7709cc107f21e3b8b0050df44e83089fb

layout/base/nsLayoutUtils.cpp

@@ -9190,6 +9190,22 @@ nsLayoutUtils::GetCumulativeApzCallbackTransform(nsIFrame* aFrame)
   return delta;
 }
 
+/* static */ nsRect
+nsLayoutUtils::ComputePartialPrerenderArea(const nsRect& aDirtyRect,
+                                           const nsRect& aOverflow,
+                                           const nsSize& aPrerenderSize)
+{
+  // 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 = aPrerenderSize.width - aDirtyRect.width;
+  nscoord yExcess = aPrerenderSize.height - aDirtyRect.height;
+  nsRect result = aDirtyRect;
+  result.Inflate(xExcess / 2, yExcess / 2);
+  return result.MoveInsideAndClamp(aOverflow);
+}
+
+
 /* static */ bool
 nsLayoutUtils::SupportsServoStyleBackend(nsIDocument* aDocument)
 {

layout/base/nsLayoutUtils.h

@@ -2875,6 +2875,19 @@ public:
    */
   static CSSPoint GetCumulativeApzCallbackTransform(nsIFrame* aFrame);
 
+  /**
+   * 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
+   * @return A rectangle that includes |aDirtyRect|, is clamped to |aOverflow|,
+   *         and is no larger than |aPrerenderSize|.
+   */
+  static nsRect ComputePartialPrerenderArea(const nsRect& aDirtyRect,
+                                            const nsRect& aOverflow,
+                                            const nsSize& aPrerenderSize);
+
   /*
    * Returns whether the given document supports being rendered with a
    * Servo-backed style system.  This checks whether Stylo is enabled

layout/painting/nsDisplayList.cpp

@@ -7184,20 +7184,7 @@ bool
 nsDisplayTransform::CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder)
 {
   return mAllowAsyncAnimation;
-}
 
-static nsRect ComputePartialPrerenderArea(const nsRect& aDirtyRect,
-                                          const nsRect& aOverflow,
-                                          const nsSize& aPrerenderSize)
-{
-  // 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 = aPrerenderSize.width - aDirtyRect.width;
-  nscoord yExcess = aPrerenderSize.height - aDirtyRect.height;
-  nsRect result = aDirtyRect;
-  result.Inflate(xExcess / 2, yExcess / 2);
-  return result.MoveInsideAndClamp(aOverflow);
 }
 
 /* static */ auto
@@ -7247,7 +7234,7 @@ nsDisplayTransform::ShouldPrerenderTransformedContent(nsDisplayListBuilder* aBui
     *aDirtyRect = overflow;
     return FullPrerender;
   } else if (gfxPrefs::PartiallyPrerenderAnimatedContent()) {
-    *aDirtyRect = ComputePartialPrerenderArea(*aDirtyRect, overflow, maxSize);
+    *aDirtyRect = nsLayoutUtils::ComputePartialPrerenderArea(*aDirtyRect, overflow, maxSize);
     return PartialPrerender;
   }