In the case where a larger area of the image is invalidated than has been decoded, use code that's logically equivalent (at least for aSY == 0) but less prone to floating point errors, and also handle constraining the src rect to the top, left, and right of the decoded rect.

This code is lifted from the nsImageGTK implementation.  Fixes one cause (hopefully all) of bug 159796, blank lines in images.  r=pinkerton, sr=sfraser, a=asa.

gfx/src/mac/nsImageMac.cpp

@@ -270,6 +270,43 @@ NS_IMETHODIMP nsImageMac::Draw(nsIRenderingContext &aContext, nsDrawingSurface a
 	if (!mImageBitsHandle)
 		return NS_ERROR_FAILURE;
 
+  PRInt32 srcWidth = aSWidth;
+  PRInt32 srcHeight = aSHeight;
+  PRInt32 dstWidth = aDWidth;
+  PRInt32 dstHeight = aDHeight;
+  PRInt32 dstOrigX = aDX;
+  PRInt32 dstOrigY = aDY;
+
+  // Ensure that the source rect is no larger than our decoded rect.
+  // If it is, clip the source rect, and scale the difference to adjust
+  // the dest rect.
+
+  PRInt32 j = aSX + aSWidth;
+  PRInt32 z;
+  if (j > mDecodedX2) {
+    z = j - mDecodedX2;
+    aDWidth -= z*dstWidth/srcWidth;
+    aSWidth -= z;
+  }
+  if (aSX < mDecodedX1) {
+    aDX += (mDecodedX1 - aSX)*dstWidth/srcWidth;
+    aSX = mDecodedX1;
+  }
+
+  j = aSY + aSHeight;
+  if (j > mDecodedY2) {
+    z = j - mDecodedY2;
+    aDHeight -= z*dstHeight/srcHeight;
+    aSHeight -= z;
+  }
+  if (aSY < mDecodedY1) {
+    aDY += (mDecodedY1 - aSY)*dstHeight/srcHeight;
+    aSY = mDecodedY1;
+  }
+
+  if (aDWidth <= 0 || aDHeight <= 0 || aSWidth <= 0 || aSHeight <= 0)
+    return NS_OK;
+
   // lock and set up bits handles
   StHandleLocker  imageBitsLocker(mImageBitsHandle);
   StHandleLocker  maskBitsLocker(mMaskBitsHandle);    // ok with nil handle
@@ -278,13 +315,6 @@ NS_IMETHODIMP nsImageMac::Draw(nsIRenderingContext &aContext, nsDrawingSurface a
   if (mMaskBitsHandle)
     mMaskPixmap.baseAddr = *mMaskBitsHandle;
 
-	// currently the top is 0, this may change and this code will have to reflect that
-	if (( mDecodedY2 < aSHeight) ) {
-		// adjust the source and dest height to reflect this
-		aDHeight = float(mDecodedY2/float(aSHeight)) * aDHeight;
-		aSHeight = mDecodedY2;
-	}	
-
 	::SetRect(&srcRect, aSX, aSY, aSX + aSWidth, aSY + aSHeight);
 	maskRect = srcRect;
 	::SetRect(&dstRect, aDX, aDY, aDX + aDWidth, aDY + aDHeight);