Bug 324896. Always paint some kind of background when we're asked to paint, for non-translucent windows. r+sr=dbaron

layout/base/nsDisplayList.h

@@ -441,6 +441,17 @@ public:
     return NS_OK;
   }
   
+  /**
+   * Append a new item to the bottom of the list. If the item is null we return
+   * NS_ERROR_OUT_OF_MEMORY. The intended usage is AppendNewToBottom(new ...);
+   */
+  nsresult AppendNewToBottom(nsDisplayItem* aItem) {
+    if (!aItem)
+      return NS_ERROR_OUT_OF_MEMORY;
+    AppendToBottom(aItem);
+    return NS_OK;
+  }
+  
   /**
    * Append a new item to the bottom of the list. The item must be non-null
    * and not already in a list.

layout/base/nsLayoutUtils.cpp

@@ -611,9 +611,42 @@ nsLayoutUtils::GetFrameForPoint(nsIFrame* aFrame, nsPoint aPt)
   return result;
 }
 
+/**
+ * A simple display item that just renders a solid color across the entire
+ * visible area.
+ */
+MOZ_DECL_CTOR_COUNTER(nsDisplaySolidColor)
+class nsDisplaySolidColor : public nsDisplayItem {
+public:
+  nsDisplaySolidColor(nsIFrame* aFrame, nscolor aColor)
+    : mFrame(aFrame), mColor(aColor) {
+    MOZ_COUNT_CTOR(nsDisplaySolidColor);
+  }
+#ifdef NS_BUILD_REFCNT_LOGGING
+  virtual ~nsDisplaySolidColor() {
+    MOZ_COUNT_DTOR(nsDisplaySolidColor);
+  }
+#endif
+
+  virtual nsIFrame* GetUnderlyingFrame() { return mFrame; }
+  virtual void Paint(nsDisplayListBuilder* aBuilder, nsIRenderingContext* aCtx,
+     const nsRect& aDirtyRect);
+  NS_DISPLAY_DECL_NAME("SolidColor")
+private:
+  nsIFrame* mFrame;
+  nscolor   mColor;
+};
+
+void nsDisplaySolidColor::Paint(nsDisplayListBuilder* aBuilder,
+     nsIRenderingContext* aCtx, const nsRect& aDirtyRect)
+{
+  aCtx->SetColor(mColor);
+  aCtx->FillRect(aDirtyRect);
+}
+
 nsresult
 nsLayoutUtils::PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFrame,
-                          const nsRegion& aDirtyRegion)
+                          const nsRegion& aDirtyRegion, nscolor aBackground)
 {
   nsDisplayListBuilder builder(aFrame, PR_FALSE);
   nsDisplayList list;
@@ -622,6 +655,16 @@ nsLayoutUtils::PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFra
     aFrame->BuildDisplayListForStackingContext(&builder, dirtyRect, &list);
   NS_ENSURE_SUCCESS(rv, rv);
 
+  if (NS_GET_A(aBackground) > 0) {
+    // Fill the visible area with a background color. In the common case,
+    // the visible area is entirely covered by the background of the root
+    // document (at least!) so this will be removed by the optimizer. In some
+    // cases we might not have a root frame, so this will prevent garbage
+    // from being drawn.
+    rv = list.AppendNewToBottom(new (&builder) nsDisplaySolidColor(aFrame, aBackground));
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
 #ifdef DEBUG
   if (gDumpPaintList) {
     fprintf(stderr, "Painting --- before optimization (dirty %d,%d,%d,%d):\n",

layout/base/nsLayoutUtils.h

@@ -329,9 +329,11 @@ public:
    * is the origin of aFrame
    * @param aDirtyRegion the region that must be painted, in the coordinates
    * of aFrame
+   * @param aBackground paint the dirty area with this color before drawing
+   * the actual content; pass NS_RGBA(0,0,0,0) to draw no background
    */
   static nsresult PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFrame,
-                             const nsRegion& aDirtyRegion);
+                             const nsRegion& aDirtyRegion, nscolor aBackground);
 
   /**
    * @param aRootFrame the root frame of the tree to be displayed

layout/base/nsPresShell.cpp

@@ -5498,28 +5498,45 @@ PresShell::Paint(nsIView*             aView,
   NS_ASSERTION(!(nsnull == aView), "null view");
 
   frame = NS_STATIC_CAST(nsIFrame*, aView->GetClientData());
-     
-  if (nsnull != frame)
-  {
-    if (mCaret)
-      mCaret->EraseCaret();
+  nscolor backgroundColor;
+  mViewManager->GetDefaultBackgroundColor(&backgroundColor);
+  nsIView* rootView;
+  mViewManager->GetRootView(rootView);
+  if (!rootView->GetParent() && rootView->HasWidget()) {
+    PRBool widgetIsTranslucent;
+    rootView->GetWidget()->GetWindowTranslucency(widgetIsTranslucent);
+    if (widgetIsTranslucent) {
+      backgroundColor = NS_RGBA(0,0,0,0);
+    }
+  }
+  
+  if (!frame) {
+    if (NS_GET_A(backgroundColor) > 0) {
+      aRenderingContext->SetColor(backgroundColor);
+      aRenderingContext->FillRect(aDirtyRegion.GetBounds());
+    }
+    return NS_OK;
+  }
+  
+  if (mCaret)
+    mCaret->EraseCaret();
 
-    nsLayoutUtils::PaintFrame(aRenderingContext, frame, aDirtyRegion);
+  nsLayoutUtils::PaintFrame(aRenderingContext, frame, aDirtyRegion,
+                            backgroundColor);
 
 #ifdef NS_DEBUG
-    // Draw a border around the frame
-    if (nsIFrameDebug::GetShowFrameBorders()) {
-      nsRect r = frame->GetRect();
-      aRenderingContext->SetColor(NS_RGB(0,0,255));
-      aRenderingContext->DrawRect(0, 0, r.width, r.height);
-    }
-    // Draw a border around the current event target
-    if ((nsIFrameDebug::GetShowEventTargetFrameBorder()) && (aView == mCurrentTargetView)) {
-      aRenderingContext->SetColor(NS_RGB(128,0,128));
-      aRenderingContext->DrawRect(mCurrentTargetRect.x, mCurrentTargetRect.y, mCurrentTargetRect.width, mCurrentTargetRect.height);
-    }
-#endif
+  // Draw a border around the frame
+  if (nsIFrameDebug::GetShowFrameBorders()) {
+    nsRect r = frame->GetRect();
+    aRenderingContext->SetColor(NS_RGB(0,0,255));
+    aRenderingContext->DrawRect(0, 0, r.width, r.height);
   }
+  // Draw a border around the current event target
+  if ((nsIFrameDebug::GetShowEventTargetFrameBorder()) && (aView == mCurrentTargetView)) {
+    aRenderingContext->SetColor(NS_RGB(128,0,128));
+    aRenderingContext->DrawRect(mCurrentTargetRect.x, mCurrentTargetRect.y, mCurrentTargetRect.width, mCurrentTargetRect.height);
+  }
+#endif
 
   return rv;
 }