Bug 791616. Part 1: Add nsIScrollableFrame::GetPerceivedScrollingDirections to consolidate logic for whether UI actions should be allowed to scroll in a given direction. r=mats

--HG--
extra : rebase_source : 470acc887036b633f43a30fb7dc6ca24ddc02dc6

content/events/src/nsEventStateManager.cpp

@@ -383,12 +383,11 @@ CanScrollOn(nsIScrollableFrame* aScrollFrame, double aDeltaX, double aDeltaY)
 
   nsPoint scrollPt = aScrollFrame->GetScrollPosition();
   nsRect scrollRange = aScrollFrame->GetScrollRange();
-  nscoord oneDevPixel =
-    aScrollFrame->GetScrolledFrame()->PresContext()->AppUnitsPerDevPixel();
+  uint32_t directions = aScrollFrame->GetPerceivedScrollingDirections();
 
-  return (aDeltaX && scrollRange.width >= oneDevPixel &&
+  return (aDeltaX && (directions & nsIScrollableFrame::HORIZONTAL) &&
           CanScrollInRange(scrollRange.x, scrollPt.x, scrollRange.XMost(), aDeltaX)) ||
-         (aDeltaY && scrollRange.height >= oneDevPixel &&
+         (aDeltaY && (directions & nsIScrollableFrame::VERTICAL) &&
           CanScrollInRange(scrollRange.y, scrollPt.y, scrollRange.YMost(), aDeltaY));
 }
 

layout/base/nsLayoutUtils.cpp

@@ -943,18 +943,12 @@ nsLayoutUtils::GetNearestScrollableFrameForDirection(nsIFrame* aFrame,
     nsIScrollableFrame* scrollableFrame = do_QueryFrame(f);
     if (scrollableFrame) {
       nsPresContext::ScrollbarStyles ss = scrollableFrame->GetScrollbarStyles();
-      uint32_t scrollbarVisibility = scrollableFrame->GetScrollbarVisibility();
-      nsRect scrollRange = scrollableFrame->GetScrollRange();
-      // Require visible scrollbars or something to scroll to in
-      // the given direction.
-      nscoord oneDevPixel = f->PresContext()->DevPixelsToAppUnits(1);
+      uint32_t directions = scrollableFrame->GetPerceivedScrollingDirections();
       if (aDirection == eVertical ?
           (ss.mVertical != NS_STYLE_OVERFLOW_HIDDEN &&
-           ((scrollbarVisibility & nsIScrollableFrame::VERTICAL) ||
-            scrollRange.height >= oneDevPixel)) :
+           (directions & nsIScrollableFrame::VERTICAL)) :
           (ss.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN &&
-           ((scrollbarVisibility & nsIScrollableFrame::HORIZONTAL) ||
-            scrollRange.width >= oneDevPixel)))
+           (directions & nsIScrollableFrame::HORIZONTAL)))
         return scrollableFrame;
     }
   }

layout/generic/nsGfxScrollFrame.cpp

@@ -4170,3 +4170,18 @@ nsGfxScrollFrameInner::FireScrolledAreaEvent()
     nsEventDispatcher::Dispatch(doc, prescontext, &event, nullptr);
   }
 }
+
+uint32_t
+nsIScrollableFrame::GetPerceivedScrollingDirections() const
+{
+  nscoord oneDevPixel = GetScrolledFrame()->PresContext()->AppUnitsPerDevPixel();
+  uint32_t directions = GetScrollbarVisibility();
+  nsRect scrollRange = GetScrollRange();
+  if (scrollRange.width >= oneDevPixel) {
+    directions |= HORIZONTAL;
+  }
+  if (scrollRange.height >= oneDevPixel) {
+    directions |= VERTICAL;
+  }
+  return directions;
+}

layout/generic/nsIScrollableFrame.h

@@ -51,6 +51,12 @@ public:
    * assumptions about scrollbar visibility.
    */
   virtual uint32_t GetScrollbarVisibility() const = 0;
+  /**
+   * Returns the directions in which scrolling is perceived to be allowed.
+   * A direction is perceived to be allowed if there is a visible scrollbar
+   * for that direction or if the scroll range is at least one device pixel.
+   */
+  uint32_t GetPerceivedScrollingDirections() const;
   /**
    * Return the actual sizes of all possible scrollbars. Returns 0 for scrollbar
    * positions that don't have a scrollbar or where the scrollbar is not visible.