Bug 1359868 - Try to pre-render offscreen portions of scrollbar thumbs. r=mstange

MozReview-Commit-ID: K3TPswpjh3O

layout/generic/nsGfxScrollFrame.cpp

@@ -3093,13 +3093,33 @@ ScrollFrameHelper::AppendScrollPartsTo(nsDisplayListBuilder*   aBuilder,
       appendToTopFlags |= APPEND_SCROLLBAR_CONTAINER;
     }
 
-    // The display port doesn't necessarily include the scrollbars, so just
-    // include all of the scrollbars if we are in a RCD-RSF. We only do
-    // this for the root scrollframe of the root content document, which is
-    // zoomable, and where the scrollbar sizes are bounded by the widget.
-    nsRect dirty = mIsRoot && mOuter->PresContext()->IsRootContentDocument()
-                   ? scrollParts[i]->GetVisualOverflowRectRelativeToParent()
-                   : aDirtyRect;
+    // The display port doesn't necessarily include the scrollbars.
+    bool thumbGetsLayer = (scrollTargetId != FrameMetrics::NULL_SCROLL_ID);
+    bool isRcdRsf = mIsRoot && mOuter->PresContext()->IsRootContentDocument();
+    nsRect dirty = aDirtyRect;
+    if (isRcdRsf || thumbGetsLayer) {
+      nsRect overflow = scrollParts[i]->GetVisualOverflowRectRelativeToParent();
+      if (isRcdRsf) {
+        // For the root content document's root scroll frame (RCD-RSF), include
+        // all of the scrollbars. We only do this for the RCD-RSF, which is
+        // zoomable, and where the scrollbar sizes are bounded by the widget.
+        dirty = overflow;
+      } else {
+        // For subframes, we still try to prerender parts of the scrollbar that
+        // are not currently visible, because they might be brought into view
+        // by async scrolling, but we bound the area to render by the size of
+        // the root reference frame (because subframe scrollbars can be
+        // arbitrary large).
+        nsSize refSize = aBuilder->RootReferenceFrame()->GetSize();
+        gfxSize scale = nsLayoutUtils::GetTransformToAncestorScale(mOuter);
+        if (scale.width != 0 && scale.height != 0) {
+          refSize.width /= scale.width;
+          refSize.height /= scale.height;
+        }
+        dirty = nsLayoutUtils::ComputePartialPrerenderArea(dirty, overflow, refSize);
+      }
+    }
+
     nsDisplayListBuilder::AutoBuildingDisplayList
       buildingForChild(aBuilder, scrollParts[i],
                        dirty + mOuter->GetOffsetTo(scrollParts[i]), true);