Bug 1508177 - Use expanded layout viewport height to tell whether we need to render the vertical overlay scrollbar. r=tnikkel

In the previous commit, we expanded layout viewport height but during reflow
the expanded layout viewport size hasn't reflected in
ScrollReflowInput::mContentsOverflowAreas.ScrollableOverflow(). We explicitly
need to use the expanded height to tell whether we need vertical vertical
scrollbars or not.

Note that the expanded layout viewport height should NOT be used for non-overlay
scrollbars cases since, for example, if the content width is specified by
`width: 100%`, the non-overlay vertical scrollbar narrows down the content's
used width a little bit because of the vertical scrollbar width, which in turn
the minimum scale gets bigger because the content's width became bit narrower,
thus the layout viewport size calculated with the minimum scale gets smaller,
then it results the vertical scrollbar no longer needs to be rendered, thus
the minimum scale gets back to the original value, then the vertical scroll
needs to be rendered again, thus this sequence of processes happens repreatedly.

Differential Revision: https://phabricator.services.mozilla.com/D40772

--HG--
extra : moz-landing-system : lando

layout/generic/nsGfxScrollFrame.cpp

@@ -473,8 +473,24 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
                std::max(0, compositionSize.height - hScrollbarDesiredHeight));
   }
 
-  nsRect scrolledRect = mHelper.GetUnsnappedScrolledRectInternal(
-      aState->mContentsOverflowAreas.ScrollableOverflow(), scrollPortSize);
+  nsRect overflowRect = aState->mContentsOverflowAreas.ScrollableOverflow();
+  // If the content height expanded by the minimum-scale will be taller than
+  // the scrollable overflow area, we need to expand the area here to tell
+  // properly whether we need to render the overlay vertical scrollbar.
+  // NOTE: This expanded size should NOT be used for non-overley scrollbars
+  // cases since putting the vertical non-overlay scrollbar will make the
+  // content width narrow a little bit, which in turn the minimum scale value
+  // becomes a bit bigger than before, then the vertical scrollbar is no longer
+  // needed, which means the content width becomes the original width, then the
+  // minimum-scale is changed to the original one, and so forth.
+  if (mHelper.UsesOverlayScrollbars() && mHelper.mIsUsingMinimumScaleSize &&
+      mHelper.mMinimumScaleSize.height > overflowRect.YMost()) {
+    MOZ_ASSERT(StaticPrefs::layout_viewport_contains_no_contents_area());
+    overflowRect.height +=
+        mHelper.mMinimumScaleSize.height - overflowRect.YMost();
+  }
+  nsRect scrolledRect =
+      mHelper.GetUnsnappedScrolledRectInternal(overflowRect, scrollPortSize);
   nscoord oneDevPixel = aState->mBoxState.PresContext()->DevPixelsToAppUnits(1);
 
   if (!aForce) {
@@ -6152,10 +6168,8 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
 
   bool hasResizer = HasResizer();
   bool scrollbarOnLeft = !IsScrollbarOnRight();
-  bool overlayScrollBars =
-      Document::UseOverlayScrollbars(presShell->GetDocument());
-  bool overlayScrollBarsWithZoom =
-      overlayScrollBars && mIsRoot && presShell->IsVisualViewportSizeSet();
+  bool overlayScrollBarsWithZoom = UsesOverlayScrollbars() && mIsRoot &&
+                                   presShell->IsVisualViewportSizeSet();
 
   nsSize scrollPortClampingSize = mScrollPort.Size();
   double res = 1.0;
@@ -6985,6 +6999,10 @@ bool ScrollFrameHelper::UsesContainerScrolling() const {
   return false;
 }
 
+bool ScrollFrameHelper::UsesOverlayScrollbars() const {
+  return Document::UseOverlayScrollbars(mOuter->PresShell()->GetDocument());
+}
+
 bool ScrollFrameHelper::DragScroll(WidgetEvent* aEvent) {
   // Dragging is allowed while within a 20 pixel border. Note that device pixels
   // are used so that the same margin is used even when zoomed in or out.

layout/generic/nsGfxScrollFrame.h

@@ -430,6 +430,8 @@ class ScrollFrameHelper : public nsIReflowCallback {
 
   bool UsesContainerScrolling() const;
 
+  bool UsesOverlayScrollbars() const;
+
   // In the case where |aDestination| is given, elements which are entirely out
   // of view when the scroll position is moved to |aDestination| are not going
   // to be used for snap positions.

layout/reftests/meta-viewport/reftest.list

@@ -20,7 +20,7 @@ pref(layout.viewport_contains_no_contents_area,true) == position-fixed-on-square
 pref(layout.viewport_contains_no_contents_area,true) fails-if(webrender) == async-scroll-to-no-content-area.html async-scroll-to-no-content-area-ref.html # bug 1571623 to track down the failure on WebRender
 pref(layout.viewport_contains_no_contents_area,true) == resolution-change-on-landscape-content.html resolution-change-on-landscape-content-ref.html
 == scrollbars-in-half-height-content.html scrollbars-in-half-height-content-ref.html
-pref(layout.viewport_contains_no_contents_area,true) fails-if(usesOverlayScrollbars) == scrollbars-in-landscape-content.html scrollbars-in-landscape-content-ref.html
+pref(layout.viewport_contains_no_contents_area,true) == scrollbars-in-landscape-content.html scrollbars-in-landscape-content-ref.html
 
 skip-if(!Android) fails-if(geckoview&&webrender) == position-fixed-on-minimum-scale-size.html position-fixed-on-minimum-scale-size-ref.html
 == position-fixed-out-of-view.html about:blank