# HG changeset patch
# Parent  91733ae6497c63c7dc5ae11ad6e4221a396c4ed5

diff --git a/layout/base/MobileViewportManager.cpp b/layout/base/MobileViewportManager.cpp
--- a/layout/base/MobileViewportManager.cpp
+++ b/layout/base/MobileViewportManager.cpp
@@ -103,9 +103,21 @@ float MobileViewportManager::ComputeIntr

   ScreenIntSize displaySize = ViewAs<ScreenPixel>(
       mDisplaySize, PixelCastJustification::LayoutDeviceIsScreenForBounds);
+  CSSSize layoutViewportSize;
+
+  nsIScrollableFrame* rootScrollableFrame =
+    mPresShell->GetRootScrollFrameAsScrollable();
+  if (!mIsFirstPaint && rootScrollableFrame) {
+    nsRect scrollableRect = nsLayoutUtils::CalculateScrollableRectForFrame(
+        rootScrollableFrame, nullptr);
+    layoutViewportSize = CSSSize::FromAppUnits(scrollableRect.Size());
+  } else {
+    layoutViewportSize = mMobileViewportSize;
+  }
+
   CSSToScreenScale intrinsicScale =
       ComputeIntrinsicScale(mDocument->GetViewportInfo(displaySize),
-                            displaySize, mMobileViewportSize);
+                            displaySize, layoutViewportSize);
   CSSToLayoutDeviceScale cssToDev =
       mPresShell->GetPresContext()->CSSToDevPixelScale();
   return (intrinsicScale / cssToDev).scale;
diff --git a/layout/generic/ViewportFrame.cpp b/layout/generic/ViewportFrame.cpp
--- a/layout/generic/ViewportFrame.cpp
+++ b/layout/generic/ViewportFrame.cpp
@@ -258,15 +258,24 @@ nsRect ViewportFrame::AdjustReflowInputA
                "We don't handle correct positioning of fixed frames with "
                "scrollbars in odd positions");

-  // Layout fixed position elements to the visual viewport size if and only if
+  // Layout fixed position elements to the layout viewport size if and only if
   // it has been set and it is larger than the computed size, otherwise use the
   // computed size.
   nsRect rect(0, 0, aReflowInput->ComputedWidth(),
               aReflowInput->ComputedHeight());
   nsIPresShell* ps = PresShell();
-  if (ps->IsVisualViewportSizeSet() &&
-      rect.Size() < ps->GetVisualViewportSize()) {
-    rect.SizeTo(ps->GetVisualViewportSize());
+  if (ps->IsVisualViewportSizeSet()) {
+    if (rect.Size() < ps->GetVisualViewportSize()) {
+      rect.SizeTo(ps->GetVisualViewportSize());
+    }
+    if (nsIScrollableFrame* rootScrollableFrame =
+        ps->GetRootScrollFrameAsScrollable()) {
+      nsRect layoutViewport = nsLayoutUtils::CalculateScrollableRectForFrame(
+          rootScrollableFrame, nullptr);
+      if (rect.Size() < layoutViewport.Size()) {
+        rect.SizeTo(layoutViewport.Size());
+      }
+    }
   }
   return rect;
 }
diff --git a/layout/painting/nsDisplayList.cpp b/layout/painting/nsDisplayList.cpp
--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -965,15 +965,25 @@ nsDisplayListBuilder::OutOfFlowDisplayDa
     // We want to ensure that fixed position elements are visible when
     // being async scrolled, so we paint them at the size of the larger
     // viewport.
-    dirtyRectRelativeToDirtyFrame =
-        nsRect(nsPoint(0, 0), aFrame->GetParent()->GetSize());
+    nsIFrame* parent = aFrame->GetParent();
+    dirtyRectRelativeToDirtyFrame = nsRect(nsPoint(0, 0), parent->GetSize());

     nsIPresShell* ps = aFrame->PresShell();
-    if (ps->IsVisualViewportSizeSet() &&
-        dirtyRectRelativeToDirtyFrame.Size() < ps->GetVisualViewportSize()) {
-      dirtyRectRelativeToDirtyFrame.SizeTo(ps->GetVisualViewportSize());
-    }
-
+    if (ps->IsVisualViewportSizeSet()) {
+      if (dirtyRectRelativeToDirtyFrame.Size() < ps->GetVisualViewportSize()) {
+        dirtyRectRelativeToDirtyFrame.SizeTo(ps->GetVisualViewportSize());
+      }
+
+      nsIScrollableFrame* scrollableFrame =
+        ps->GetRootScrollFrameAsScrollable();
+      if (scrollableFrame) {
+        nsRect layoutViewport = nsLayoutUtils::CalculateScrollableRectForFrame(
+            scrollableFrame, nullptr);
+        if (dirtyRectRelativeToDirtyFrame.Size() < layoutViewport.Size()) {
+          dirtyRectRelativeToDirtyFrame.SizeTo(layoutViewport.Size());
+        }
+      }
+    }
     visible = dirtyRectRelativeToDirtyFrame;
   }
 #endif