Issue #146 - Part 3: Create nsDisplayTableBackgroundImage to avoid

display list collisions when processing the background image of a table.

layout/base/nsDisplayList.h

@@ -2814,6 +2814,8 @@ protected:
   void PaintInternal(nsDisplayListBuilder* aBuilder, nsRenderingContext* aCtx,
                      const nsRect& aBounds, nsRect* aClipRect);
 
+  virtual nsIFrame* StyleFrame() { return mFrame; }
+
   // Determine whether we want to be separated into our own layer, independent
   // of whether this item can actually be layerized.
   enum ImageLayerization {
@@ -2859,6 +2861,41 @@ static_assert(
   "TableType cannot fit with TableTypeBits::COUNT");
 TableType GetTableTypeFromFrame(nsIFrame* aFrame);
 
+/**
+ * A display item to paint background image for table. For table parts, such
+ * as row, row group, col, col group, when drawing its background, we'll
+ * create separate background image display item for its containning cell.
+ * Those background image display items will reference to same DisplayItemData
+ * if we keep the mFrame point to cell's ancestor frame. We don't want to this
+ * happened bacause share same DisplatItemData will cause many bugs. So that
+ * we let mFrame point to cell frame and store the table type of the ancestor
+ * frame. And use mFrame and table type as key to generate DisplayItemData to
+ * avoid sharing DisplayItemData.
+ *
+ * Also store ancestor frame as mStyleFrame for all rendering informations.
+ */
+class nsDisplayTableBackgroundImage : public nsDisplayBackgroundImage {
+public:
+  nsDisplayTableBackgroundImage(nsDisplayListBuilder* aBuilder,
+                                nsIFrame* aFrame,
+                                uint32_t aLayer,
+                                const nsRect& aBackgroundRect,
+                                const nsStyleBackground* aBackgroundStyle,
+                                nsIFrame* aCellFrame);
+
+  virtual uint32_t GetPerFrameKey() override {
+    return (static_cast<uint8_t>(mTableType) << nsDisplayItem::TYPE_BITS) |
+           nsDisplayItem::GetPerFrameKey();
+  }
+
+  virtual bool IsInvalid(nsRect& aRect) override;
+protected:
+  virtual nsIFrame* StyleFrame() override { return mStyleFrame; }
+
+  nsIFrame* mStyleFrame;
+  TableType mTableType;
+};
+
 /**
  * A display item to paint the native theme background for a frame.
  */