bug 380185 (part 2) - Scrollbars should not try to draw their features if there is not enough space. r=josh sr=roc.

widget/src/cocoa/nsNativeThemeCocoa.mm

@@ -402,6 +402,8 @@ nsNativeThemeCocoa::GetScrollbarPressStates(nsIFrame *aFrame, PRInt32 aButtonSta
 }
 
 
+// Both of the following sets of numbers were derived by loading the testcase in
+// bmo bug 380185 in Safari and observing its behavior for various heights of scrollbar.
 // These magic numbers are the minimum sizes we can draw a scrollbar and still 
 // have room for everything to display, including the thumb
 #define MIN_SCROLLBAR_SIZE_WITH_THUMB 61
@@ -432,20 +434,19 @@ nsNativeThemeCocoa::GetScrollbarDrawInfo(HIThemeTrackDrawInfo& aTdi, nsIFrame *a
   if (isHorizontal)
     aTdi.attributes |= kThemeTrackHorizontal;
 
-  aTdi.trackInfo.scrollbar.viewsize = (SInt32)(isHorizontal ? (aRect.size.width) : (aRect.size.height));
+  PRInt32 longSideLength = (PRInt32)(isHorizontal ? (aRect.size.width) : (aRect.size.height));
+  aTdi.trackInfo.scrollbar.viewsize = (SInt32)longSideLength;
 
   // Only display the thumb if we have room for it to display. Note that this doesn't 
   // affect the actual tracking rects Gecko maintains -- this is a purely cosmetic
   // change. See bmo bug 380185 for more info.
-  if ((isHorizontal ? aRect.size.width : aRect.size.height) >=
-      (isSmall ? MIN_SMALL_SCROLLBAR_SIZE_WITH_THUMB : MIN_SCROLLBAR_SIZE_WITH_THUMB)) {
+  if (longSideLength >= (isSmall ? MIN_SMALL_SCROLLBAR_SIZE_WITH_THUMB : MIN_SCROLLBAR_SIZE_WITH_THUMB)) {
     aTdi.attributes |= kThemeTrackShowThumb;
   }
   // If we don't have enough room to display *any* features, we're done creating 
   // this tdi, so return early. Again, this doesn't affect the tracking rects Gecko 
   // maintains. See bmo bug 380185 for more info.
-  else if ((isHorizontal ? aRect.size.width : aRect.size.height) < 
-           (isSmall ? MIN_SMALL_SCROLLBAR_SIZE : MIN_SCROLLBAR_SIZE)) {
+  else if (longSideLength < (isSmall ? MIN_SMALL_SCROLLBAR_SIZE : MIN_SCROLLBAR_SIZE)) {
     aTdi.enableState = kThemeTrackNothingToScroll;
     return;
   }