Bug 791616. Part 2: Add API to ScrollIntoView methods to control whether to scroll in a direction when that direction is not a perceived scrollable direction. r=mats

layout/base/nsIPresShell.h

@@ -568,7 +568,8 @@ public:
   };
   typedef struct ScrollAxis {
     int16_t mWhereToScroll;
-    WhenToScroll mWhenToScroll : 16;
+    WhenToScroll mWhenToScroll : 8;
+    bool mOnlyIfPerceivedScrollableDirection : 1;
   /**
    * @param aWhere: Either a percentage or a special value.
    *                nsIPresShell defines:
@@ -599,10 +600,17 @@ public:
    *                is visible.
    *                * SCROLL_ALWAYS: Move the frame regardless of its current
    *                visibility.
+   * @param aOnlyIfPerceivedScrollableDirection:
+   *                If the direction is not a perceived scrollable direction (i.e.
+   *                no scrollbar showing and less than one device pixel of
+   *                scrollable distance), don't scroll. Defaults to false.
    */
     ScrollAxis(int16_t aWhere = SCROLL_MINIMUM,
-               WhenToScroll aWhen = SCROLL_IF_NOT_FULLY_VISIBLE) :
-                 mWhereToScroll(aWhere), mWhenToScroll(aWhen) {}
+               WhenToScroll aWhen = SCROLL_IF_NOT_FULLY_VISIBLE,
+               bool aOnlyIfPerceivedScrollableDirection = false) :
+      mWhereToScroll(aWhere), mWhenToScroll(aWhen),
+      mOnlyIfPerceivedScrollableDirection(aOnlyIfPerceivedScrollableDirection)
+    {}
   } ScrollAxis;
   /**
    * Scrolls the view of the document so that the primary frame of the content

layout/base/nsPresShell.cpp

@@ -3106,9 +3106,12 @@ static void ScrollToShowRect(nsIScrollableFrame*      aScrollFrame,
   nsPresContext::ScrollbarStyles ss = aScrollFrame->GetScrollbarStyles();
   nsRect allowedRange(scrollPt, nsSize(0, 0));
   bool needToScroll = false;
+  uint32_t directions = aScrollFrame->GetPerceivedScrollingDirections();
 
-  if ((aFlags & nsIPresShell::SCROLL_OVERFLOW_HIDDEN) ||
-      ss.mVertical != NS_STYLE_OVERFLOW_HIDDEN) {
+  if (((aFlags & nsIPresShell::SCROLL_OVERFLOW_HIDDEN) ||
+       ss.mVertical != NS_STYLE_OVERFLOW_HIDDEN) &&
+      (!aVertical.mOnlyIfPerceivedScrollableDirection ||
+       (directions & nsIScrollableFrame::VERTICAL))) {
 
     if (ComputeNeedToScroll(aVertical.mWhenToScroll,
                             lineSize.height,
@@ -3129,8 +3132,10 @@ static void ScrollToShowRect(nsIScrollableFrame*      aScrollFrame,
     }
   }
 
-  if ((aFlags & nsIPresShell::SCROLL_OVERFLOW_HIDDEN) ||
-      ss.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN) {
+  if (((aFlags & nsIPresShell::SCROLL_OVERFLOW_HIDDEN) ||
+       ss.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN) &&
+      (!aHorizontal.mOnlyIfPerceivedScrollableDirection ||
+       (directions & nsIScrollableFrame::HORIZONTAL))) {
 
     if (ComputeNeedToScroll(aHorizontal.mWhenToScroll,
                             lineSize.width,