Bug 1648500 - Use the composition size when positioning scrollbars. r=tnikkel

When positioning overlay scrollbars, the code was previously taking the
visual viewport size and scaling it back up to the composition size using
the resolution. This works because overlay scrollbars don't take up any
space, and so the visual viewport size is exactly equal to the composition
size divided by the resolution. However, it's simpler to just use the
composition size because we can get that easily enough. And this fixes the
scrollbar positioning on the very first reflow, before the visual viewport
has been set on the presShell.

Eventually this might make it easier to do this for non-overlay scrollbars
as well, since those do take up layout space, and can't be positioned using
the "VV size multiplied by resolution" quantity, as that doesn't exactly
equal the composition size.

Differential Revision: https://phabricator.services.mozilla.com/D81277

layout/generic/nsGfxScrollFrame.cpp

@@ -6290,18 +6290,14 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
                                          const nsRect& aOldScrollArea) {
   NS_ASSERTION(!mSuppressScrollbarUpdate, "This should have been suppressed");
 
-  PresShell* presShell = mOuter->PresShell();
-
   bool hasResizer = HasResizer();
   bool scrollbarOnLeft = !IsScrollbarOnRight();
-  bool overlayScrollBarsWithZoom = UsesOverlayScrollbars() && mIsRoot &&
-                                   presShell->IsVisualViewportSizeSet();
+  bool overlayScrollBarsOnRoot = UsesOverlayScrollbars() && mIsRoot;
 
-  nsSize scrollPortClampingSize = mScrollPort.Size();
-  double res = 1.0;
-  if (overlayScrollBarsWithZoom) {
-    scrollPortClampingSize = presShell->GetVisualViewportSize();
-    res = presShell->GetCumulativeResolution();
+  nsSize compositionSize = mScrollPort.Size();
+  if (overlayScrollBarsOnRoot) {
+    compositionSize = nsLayoutUtils::CalculateCompositionSizeForFrame(
+        mOuter, false, &compositionSize);
   }
 
   // place the scrollcorner
@@ -6370,14 +6366,12 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
   if (mVScrollbarBox) {
     MOZ_ASSERT(mVScrollbarBox->IsXULBoxFrame(), "Must be a box frame!");
     vRect = mScrollPort;
-    if (overlayScrollBarsWithZoom) {
-      vRect.height = NSToCoordRound(res * scrollPortClampingSize.height);
+    if (overlayScrollBarsOnRoot) {
+      vRect.height = compositionSize.height;
     }
     vRect.width = aContentArea.width - mScrollPort.width;
-    vRect.x = scrollbarOnLeft
-                  ? aContentArea.x
-                  : mScrollPort.x +
-                        NSToCoordRound(res * scrollPortClampingSize.width);
+    vRect.x = scrollbarOnLeft ? aContentArea.x
+                              : mScrollPort.x + compositionSize.width;
     if (mHasVerticalScrollbar) {
       nsMargin margin;
       mVScrollbarBox->GetXULMargin(margin);
@@ -6391,12 +6385,11 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
   if (mHScrollbarBox) {
     MOZ_ASSERT(mHScrollbarBox->IsXULBoxFrame(), "Must be a box frame!");
     hRect = mScrollPort;
-    if (overlayScrollBarsWithZoom) {
-      hRect.width = NSToCoordRound(res * scrollPortClampingSize.width);
+    if (overlayScrollBarsOnRoot) {
+      hRect.width = compositionSize.width;
     }
     hRect.height = aContentArea.height - mScrollPort.height;
-    hRect.y =
-        mScrollPort.y + NSToCoordRound(res * scrollPortClampingSize.height);
+    hRect.y = mScrollPort.y + compositionSize.height;
     if (mHasHorizontalScrollbar) {
       nsMargin margin;
       mHScrollbarBox->GetXULMargin(margin);