Bug 257612. Fix dynamic margin changes by 1) Propagate IsPreviousMarginDirty through empty lines 2) Reflow any line whose previous margin is dirty. 3) If a reflowed line could have been tested in a subsequent line's ShouldApplyTopMargin both before and after reflow, and might have been empty before or after reflow, then make the next line's previous margin dirty. 4) If reflowing a block changes its carried-out margin, then mark the next line's previous margin dirty. r+sr=dbaron

layout/generic/nsHTMLReflowMetrics.h

@@ -79,6 +79,17 @@ struct nsCollapsingMargin {
       {
       }
 
+    PRBool operator==(const nsCollapsingMargin& aOther)
+      {
+        return mMostPos == aOther.mMostPos &&
+          mMostNeg == aOther.mMostNeg;
+      }
+
+    PRBool operator!=(const nsCollapsingMargin& aOther)
+      {
+        return !(*this == aOther);
+      }
+
     nsCollapsingMargin& operator=(const nsCollapsingMargin& aOther)
       {
         mMostPos = aOther.mMostPos;