Bug 1042151 Part 2 - Refactor the conditions that decide which edge to clamp when getting scrolled rect. r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D86076

layout/base/nsLayoutUtils.cpp

@@ -2219,13 +2219,19 @@ nsRect nsLayoutUtils::GetScrolledRect(nsIFrame* aScrolledFrame,
           y1 = aScrolledFrameOverflowArea.y,
           y2 = aScrolledFrameOverflowArea.YMost();
 
-  bool horizontal = !wm.IsVertical();
+  const bool isHorizontalWM = !wm.IsVertical();
+  const bool isVerticalWM = wm.IsVertical();
+  bool isInlineFlowFromTopOrLeft = !wm.IsInlineReversed();
+  bool isBlockFlowFromTopOrLeft = isHorizontalWM || wm.IsVerticalLR();
+
+  // TODO: Consider flexbox's main-axis and cross-axis in
+  // isInlineFlowFromTopOrLeft and isBlockFlowFromTopOrLeft.
 
   // Clamp the horizontal start-edge (x1 or x2, depending whether the logical
-  // axis that corresponds to horizontal progresses from L-R or R-L).
-  // In horizontal writing mode, we need to check IsInlineReversed() to see
-  // which side to clamp; in vertical mode, it depends on the block direction.
-  if ((horizontal && !wm.IsInlineReversed()) || wm.IsVerticalLR()) {
+  // axis that corresponds to horizontal progresses from left-to-right or
+  // right-to-left).
+  if ((isHorizontalWM && isInlineFlowFromTopOrLeft) ||
+      (isVerticalWM && isBlockFlowFromTopOrLeft)) {
     if (x1 < 0) {
       x1 = 0;
     }
@@ -2245,10 +2251,11 @@ nsRect nsLayoutUtils::GetScrolledRect(nsIFrame* aScrolledFrame,
     x2 += extraWidth;
   }
 
-  // Similarly, clamp the vertical start-edge.
-  // In horizontal writing mode, the block direction is always top-to-bottom;
-  // in vertical writing mode, we need to check IsInlineReversed().
-  if (horizontal || !wm.IsInlineReversed()) {
+  // Similarly, clamp the vertical start-edge (y1 or y2, depending whether the
+  // logical axis that corresponds to vertical progresses from top-to-bottom or
+  // buttom-to-top).
+  if ((isHorizontalWM && isBlockFlowFromTopOrLeft) ||
+      (isVerticalWM && isInlineFlowFromTopOrLeft)) {
     if (y1 < 0) {
       y1 = 0;
     }