Make sure scrollcorners are always boxes. Bug 310919, r=dbaron, sr=neil

layout/generic/nsGfxScrollFrame.cpp

@@ -2369,6 +2369,7 @@ nsGfxScrollFrameInner::LayoutScrollbars(nsBoxLayoutState& aState,
   scrollable->SetLineHeight(fontHeight);
 
   if (mVScrollbarBox) {
+    NS_PRECONDITION(mVScrollbarBox->IsBoxFrame(), "Must be a box frame!");
     if (!mHasVerticalScrollbar) {
       SetAttribute(mVScrollbarBox, nsXULAtoms::curpos, 0);
     }
@@ -2387,6 +2388,7 @@ nsGfxScrollFrameInner::LayoutScrollbars(nsBoxLayoutState& aState,
   }
     
   if (mHScrollbarBox) {
+    NS_PRECONDITION(mHScrollbarBox->IsBoxFrame(), "Must be a box frame!");
     if (!mHasHorizontalScrollbar) {
       SetAttribute(mHScrollbarBox, nsXULAtoms::curpos, 0);
     }
@@ -2406,6 +2408,7 @@ nsGfxScrollFrameInner::LayoutScrollbars(nsBoxLayoutState& aState,
 
   // place the scrollcorner
   if (mScrollCornerBox) {
+    NS_PRECONDITION(mScrollCornerBox->IsBoxFrame(), "Must be a box frame!");
     nsRect r(0, 0, 0, 0);
     if (aContentArea.x != aScrollArea.x) {
       // scrollbar (if any) on left

toolkit/content/xul.css

@@ -867,6 +867,14 @@ scrollbar, scrollbarbutton, scrollcorner, slider, thumb {
   -moz-user-select: none;
 }
 
+scrollcorner {
+  display: -moz-box !important;
+}
+
+scrollcorner[hidden="true"] {
+  display: none !important;
+}
+
 scrollbar[value="hidden"] {
   visibility: hidden;
 }

xpfe/global/resources/content/xul.css

@@ -760,6 +760,14 @@ scrollbar, scrollbarbutton, scrollcorner, slider, thumb {
   -moz-user-select: none;
 }
 
+scrollcorner {
+  display: -moz-box !important;
+}
+
+scrollcorner[hidden="true"] {
+  display: none !important;
+}
+
 scrollbar[value="hidden"] {
   visibility: hidden;
 }