Bug 486065. Hack nsCSSOffsetState::InitOffsets to zero out computed padding and borders when the scrollbar size is zero. r=dbaron

--HG--
extra : rebase_source : 92bcaee46ded00722e3fc86b4ca538e5f90f16ef

layout/generic/nsHTMLReflowState.cpp

@@ -1925,7 +1925,8 @@ nsCSSOffsetState::InitOffsets(nscoord aContainingBlockWidth,
   }
   mComputedBorderPadding += mComputedPadding;
 
-  if (frame->GetType() == nsGkAtoms::tableFrame) {
+  nsIAtom* frameType = frame->GetType();
+  if (frameType == nsGkAtoms::tableFrame) {
     nsTableFrame *tableFrame = static_cast<nsTableFrame*>(frame);
 
     if (tableFrame->IsBorderCollapse()) {
@@ -1936,6 +1937,23 @@ nsCSSOffsetState::InitOffsets(nscoord aContainingBlockWidth,
       mComputedPadding.SizeTo(0,0,0,0);
       mComputedBorderPadding = tableFrame->GetIncludedOuterBCBorder();
     }
+  } else if (frameType == nsGkAtoms::scrollbarFrame) {
+    // scrollbars may have had their width or height smashed to zero
+    // by the associated scrollframe, in which case we must not report
+    // any padding or border on that axis.
+    nsSize size(frame->GetSize());
+    if (size.width == 0) {
+      mComputedPadding.left = 0;
+      mComputedPadding.right = 0;
+      mComputedBorderPadding.left = 0;
+      mComputedBorderPadding.right = 0;
+    }
+    if (size.height == 0) {
+      mComputedPadding.top = 0;
+      mComputedPadding.bottom = 0;
+      mComputedBorderPadding.top = 0;
+      mComputedBorderPadding.bottom = 0;
+    }
   }
 }