Bug 926292 - Put overlay scrollbars on top of the topmost positioned descendant of the scrolled frame. r=mats

layout/base/nsDisplayList.cpp

@@ -1637,9 +1637,7 @@ static bool IsZOrderLEQ(nsDisplayItem* aItem1, nsDisplayItem* aItem2,
                         void* aClosure) {
   // Note that we can't just take the difference of the two
   // z-indices here, because that might overflow a 32-bit int.
-  int32_t index1 = nsLayoutUtils::GetZIndex(aItem1->Frame());
-  int32_t index2 = nsLayoutUtils::GetZIndex(aItem2->Frame());
-  return index1 <= index2;
+  return aItem1->ZIndex() <= aItem2->ZIndex();
 }
 
 void nsDisplayList::SortByZOrder(nsDisplayListBuilder* aBuilder,
@@ -1699,6 +1697,20 @@ nsDisplayItem::MaxActiveLayers()
   return sMaxLayers;
 }
 
+int32_t
+nsDisplayItem::ZIndex() const
+{
+  if (!mFrame->IsPositioned() && !mFrame->IsFlexItem())
+    return 0;
+
+  const nsStylePosition* position = mFrame->StylePosition();
+  if (position->mZIndex.GetUnit() == eStyleUnit_Integer)
+    return position->mZIndex.GetIntValue();
+
+  // sort the auto and 0 elements together
+  return 0;
+}
+
 bool
 nsDisplayItem::RecomputeVisibility(nsDisplayListBuilder* aBuilder,
                                    nsRegion* aVisibleRegion) {
@@ -2958,7 +2970,9 @@ nsDisplayBoxShadowInner::ComputeVisibility(nsDisplayListBuilder* aBuilder,
 
 nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayList* aList)
-  : nsDisplayItem(aBuilder, aFrame) {
+  : nsDisplayItem(aBuilder, aFrame)
+  , mOverrideZIndex(0)
+{
   mList.AppendToTop(aList);
   UpdateBounds(aBuilder);
 
@@ -3000,7 +3014,9 @@ nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
 
 nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayItem* aItem)
-  : nsDisplayItem(aBuilder, aFrame) {
+  : nsDisplayItem(aBuilder, aFrame)
+  , mOverrideZIndex(0)
+{
   mList.AppendToTop(aItem);
   UpdateBounds(aBuilder);
   
@@ -3026,7 +3042,9 @@ nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayItem* aItem,
                                      const nsIFrame* aReferenceFrame,
                                      const nsPoint& aToReferenceFrame)
-  : nsDisplayItem(aBuilder, aFrame, aReferenceFrame, aToReferenceFrame) {
+  : nsDisplayItem(aBuilder, aFrame, aReferenceFrame, aToReferenceFrame)
+  , mOverrideZIndex(0)
+{
   mList.AppendToTop(aItem);
   mBounds = mList.GetBounds(aBuilder);
 }

layout/base/nsDisplayList.h

@@ -840,6 +840,12 @@ public:
    * returns null.
    */
   inline nsIFrame* Frame() const { return mFrame; }
+  /**
+   * Compute the used z-index of our frame; returns zero for elements to which
+   * z-index does not apply, and for z-index:auto.
+   * @note This can be overridden, @see nsDisplayWrapList::SetOverrideZIndex.
+   */
+  virtual int32_t ZIndex() const;
   /**
    * The default bounds is the frame border rect.
    * @param aSnap *aSnap is set to true if the returned rect will be
@@ -2447,7 +2453,7 @@ public:
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                     nsDisplayItem* aItem, const nsIFrame* aReferenceFrame, const nsPoint& aToReferenceFrame);
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
-    : nsDisplayItem(aBuilder, aFrame) {}
+    : nsDisplayItem(aBuilder, aFrame), mOverrideZIndex(0) {}
   virtual ~nsDisplayWrapList();
   /**
    * Call this if the wrapped list is changed.
@@ -2506,6 +2512,16 @@ public:
   }
   virtual nsDisplayList* GetChildren() MOZ_OVERRIDE { return &mList; }
 
+  virtual int32_t ZIndex() const MOZ_OVERRIDE
+  {
+    return (mOverrideZIndex > 0) ? mOverrideZIndex : nsDisplayItem::ZIndex();
+  }
+
+  void SetOverrideZIndex(int32_t aZIndex)
+  {
+    mOverrideZIndex = aZIndex;
+  }
+
   /**
    * This creates a copy of this item, but wrapping aItem instead of
    * our existing list. Only gets called if this item returned nullptr
@@ -2549,6 +2565,8 @@ protected:
   // this item's own frame.
   nsTArray<nsIFrame*> mMergedFrames;
   nsRect mBounds;
+  // Overrides the ZIndex of our frame if > 0.
+  int32_t mOverrideZIndex;
 };
 
 /**

layout/base/nsLayoutUtils.cpp

@@ -2496,20 +2496,6 @@ nsLayoutUtils::PaintFrame(nsRenderingContext* aRenderingContext, nsIFrame* aFram
   return NS_OK;
 }
 
-int32_t
-nsLayoutUtils::GetZIndex(nsIFrame* aFrame) {
-  if (!aFrame->IsPositioned() && !aFrame->IsFlexItem())
-    return 0;
-
-  const nsStylePosition* position =
-    aFrame->StylePosition();
-  if (position->mZIndex.GetUnit() == eStyleUnit_Integer)
-    return position->mZIndex.GetIntValue();
-
-  // sort the auto and 0 elements together
-  return 0;
-}
-
 /**
  * Uses a binary search for find where the cursor falls in the line of text
  * It also keeps track of the part of the string that has already been measured

layout/base/nsLayoutUtils.h

@@ -841,12 +841,6 @@ public:
                              const nsRegion& aDirtyRegion, nscolor aBackstop,
                              uint32_t aFlags = 0);
 
-  /**
-   * Compute the used z-index of aFrame; returns zero for elements to which
-   * z-index does not apply, and for z-index:auto
-   */
-  static int32_t GetZIndex(nsIFrame* aFrame);
-
   /**
    * Uses a binary search for find where the cursor falls in the line of text
    * It also keeps track of the part of the string that has already been measured

layout/generic/nsFrame.cpp

@@ -2024,13 +2024,10 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   // 3: negative z-index children.
   for (;;) {
     nsDisplayItem* item = set.PositionedDescendants()->GetBottom();
-    if (item) {
-      nsIFrame* f = item->Frame();
-      if (nsLayoutUtils::GetZIndex(f) < 0) {
-        set.PositionedDescendants()->RemoveBottom();
-        resultList.AppendToTop(item);
-        continue;
-      }
+    if (item && item->ZIndex() < 0) {
+      set.PositionedDescendants()->RemoveBottom();
+      resultList.AppendToTop(item);
+      continue;
     }
     break;
   }

layout/generic/nsGfxScrollFrame.cpp

@@ -2092,19 +2092,39 @@ ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange, nsIAtom* aOri
   }
 }
 
+static int32_t
+MaxZIndexInList(nsDisplayList* aList, nsDisplayListBuilder* aBuilder)
+{
+  int32_t maxZIndex = 0;
+  for (nsDisplayItem* item = aList->GetBottom(); item; item = item->GetAbove()) {
+    maxZIndex = std::max(maxZIndex, item->ZIndex());
+  }
+  return maxZIndex;
+}
+
 static void
-AppendToTop(nsDisplayListBuilder* aBuilder, nsDisplayList* aDest,
+AppendToTop(nsDisplayListBuilder* aBuilder, const nsDisplayListSet& aLists,
             nsDisplayList* aSource, nsIFrame* aSourceFrame, bool aOwnLayer,
-            uint32_t aFlags, mozilla::layers::FrameMetrics::ViewID aScrollTargetId)
+            uint32_t aFlags, mozilla::layers::FrameMetrics::ViewID aScrollTargetId,
+            bool aPositioned)
 {
   if (aSource->IsEmpty())
     return;
-  if (aOwnLayer) {
-    aDest->AppendNewToTop(
-        new (aBuilder) nsDisplayOwnLayer(aBuilder, aSourceFrame, aSource,
-                                         aFlags, aScrollTargetId));
+
+  nsDisplayWrapList* newItem = aOwnLayer?
+    new (aBuilder) nsDisplayOwnLayer(aBuilder, aSourceFrame, aSource,
+                                     aFlags, aScrollTargetId) :
+    new (aBuilder) nsDisplayWrapList(aBuilder, aSourceFrame, aSource);
+
+  nsDisplayList* positionedDescendants = aLists.PositionedDescendants();
+  if (aPositioned && !positionedDescendants->IsEmpty()) {
+    // We want overlay scrollbars to always be on top of the scrolled content,
+    // but we don't want them to unnecessarily cover overlapping elements from
+    // outside our scroll frame.
+    newItem->SetOverrideZIndex(MaxZIndexInList(positionedDescendants, aBuilder));
+    positionedDescendants->AppendNewToTop(newItem);
   } else {
-    aDest->AppendToTop(aSource);
+    aLists.BorderBackground()->AppendNewToTop(newItem);
   }
 }
 
@@ -2163,14 +2183,6 @@ ScrollFrameHelper::AppendScrollPartsTo(nsDisplayListBuilder*   aBuilder,
       aBuilder, scrollParts[i], aDirtyRect, partList,
       nsIFrame::DISPLAY_CHILD_FORCE_STACKING_CONTEXT);
 
-    // Don't append textarea resizers to the positioned descendants because
-    // we don't want them to float on top of overlapping elements.
-    bool appendToPositioned = aPositioned &&
-                              !(scrollParts[i] == mResizerBox && !mIsRoot);
-
-    nsDisplayList* dest = appendToPositioned ?
-      aLists.PositionedDescendants() : aLists.BorderBackground();
-
     uint32_t flags = 0;
     if (scrollParts[i] == mVScrollbarBox) {
       flags |= nsDisplayOwnLayer::VERTICAL_SCROLLBAR;
@@ -2181,9 +2193,9 @@ ScrollFrameHelper::AppendScrollPartsTo(nsDisplayListBuilder*   aBuilder,
 
     // DISPLAY_CHILD_FORCE_STACKING_CONTEXT put everything into
     // partList.PositionedDescendants().
-    ::AppendToTop(aBuilder, dest,
+    ::AppendToTop(aBuilder, aLists,
                   partList.PositionedDescendants(), scrollParts[i],
-                  aCreateLayer, flags, scrollTargetId);
+                  aCreateLayer, flags, scrollTargetId, aPositioned);
   }
 }
 
@@ -2601,8 +2613,6 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
       aBuilder, mScrolledFrame, mOuter);
     scrolledContent.BorderBackground()->AppendNewToBottom(layerItem);
   }
-  scrolledContent.MoveTo(aLists);
-
   // Now display overlay scrollbars and the resizer, if we have one.
 #ifdef MOZ_WIDGET_GONK
   // TODO: only layerize the overlay scrollbars if this scrollframe can be
@@ -2610,8 +2620,9 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   // that's where we want the layerized scrollbars
   createLayersForScrollbars = true;
 #endif
-  AppendScrollPartsTo(aBuilder, aDirtyRect, aLists, createLayersForScrollbars,
-                      true);
+  AppendScrollPartsTo(aBuilder, aDirtyRect, scrolledContent,
+                      createLayersForScrollbars, true);
+  scrolledContent.MoveTo(aLists);
 }
 
 bool

layout/reftests/z-index/overlayscrollbar-sorting-1.html

@@ -0,0 +1,26 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test that overlay scrollbars are on top of positioned content</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+}
+
+#content {
+  height: 400px;
+  background: cyan;
+  position: relative;
+  z-index: 1;
+}
+
+</style>
+
+<div id="outer"><div id="content"></div></div>

layout/reftests/z-index/overlayscrollbar-sorting-2.html

@@ -0,0 +1,32 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test that overlay scrollbars are covered by overlapping non-positioned siblings</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+  background: cyan;
+}
+
+#content {
+  height: 400px;
+}
+
+#cover {
+  margin-top: -200px;
+  width: 200px;
+  height: 200px;
+  background: cyan;
+}
+
+</style>
+
+<div id="outer"><div id="content"></div></div>
+<div id="cover"></div>

layout/reftests/z-index/overlayscrollbar-sorting-3.html

@@ -0,0 +1,33 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test that overlay scrollbars are covered by overlapping positioned siblings</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+  background: cyan;
+}
+
+#content {
+  height: 400px;
+}
+
+#cover {
+  margin-top: -200px;
+  width: 200px;
+  height: 200px;
+  background: cyan;
+  position: relative;
+}
+
+</style>
+
+<div id="outer"><div id="content"></div></div>
+<div id="cover"></div>

layout/reftests/z-index/overlayscrollbar-sorting-4.html

@@ -0,0 +1,35 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test that overlay scrollbars are covered by positioned siblings with higher z-index even when the scrollable frame has a positioned descendant</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+}
+
+#content {
+  height: 400px;
+  background: cyan;
+  position: relative;
+  z-index: 1;
+}
+
+#cover {
+  width: 200px;
+  height: 200px;
+  background: cyan;
+  position: absolute;
+  z-index: 2;
+}
+
+</style>
+
+<div id="cover"></div>
+<div id="outer"><div id="content"></div></div>

layout/reftests/z-index/overlayscrollbar-sorting-5.html

@@ -0,0 +1,43 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Test that overlay scrollbars are on top of positioned siblings when the scrollable frame has a positioned descendant that's higher than the sibling</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+}
+
+#content {
+  height: 400px;
+  background: cyan;
+  position: relative;
+  z-index: 3;
+}
+
+/* This test has different behavior depending on the type of scrollbar used.
+ * We want the scrollbar to be visible. When overlay scrollbars are used,
+ * they should be visible even when #cover gets between #outer and #content,
+ * but for non-overlay scrollbars, that would cover them so we disable the
+ * cover.
+ */
+@media all and (-moz-overlay-scrollbars) {
+  #cover {
+    width: 200px;
+    height: 200px;
+    background: cyan;
+    position: absolute;
+    z-index: 2;
+  }
+}
+
+</style>
+
+<div id="cover"></div>
+<div id="outer"><div id="content"></div></div>

layout/reftests/z-index/overlayscrollbar-sorting-ref-hidden.html

@@ -0,0 +1,24 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Reference without scrollbar</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+  background: cyan;
+}
+
+#content {
+  height: 0px;
+}
+
+</style>
+
+<div id="outer"><div id="content"></div></div>

layout/reftests/z-index/overlayscrollbar-sorting-ref-visible.html

@@ -0,0 +1,24 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Reference with scrollbar</title>
+
+<style>
+
+#outer {
+  width: 200px;
+  height: 200px;
+  overflow: auto;
+  background: cyan;
+}
+
+#content {
+  height: 400px;
+}
+
+</style>
+
+<div id="outer"><div id="content"></div></div>

layout/reftests/z-index/reftest.list

@@ -1,5 +1,15 @@
+# Make overlay scrollbars never fade out
+default-preferences pref(layout.testing.overlay-scrollbars.always-visible,true)
+
 == 480053-1.html 480053-1-ref.html
 == z-index-1.html z-index-1-ref.html
 != stacking-context-yes.html stacking-context-no.html
 == stacking-context-perspective.html stacking-context-yes.html
 == stacking-context-backface-visibility.html stacking-context-no.html
+
+fails-if(Android) != overlayscrollbar-sorting-ref-visible.html overlayscrollbar-sorting-ref-hidden.html
+== overlayscrollbar-sorting-1.html overlayscrollbar-sorting-ref-visible.html
+== overlayscrollbar-sorting-2.html overlayscrollbar-sorting-ref-hidden.html
+== overlayscrollbar-sorting-3.html overlayscrollbar-sorting-ref-hidden.html
+== overlayscrollbar-sorting-4.html overlayscrollbar-sorting-ref-hidden.html
+== overlayscrollbar-sorting-5.html overlayscrollbar-sorting-ref-visible.html