From 9207223d59e4af08d385dfd94306ec41b25619ee Mon Sep 17 00:00:00 2001
From: Bas Schouten <bschouten@mozilla.com>
Date: Thu, 17 Aug 2017 15:45:31 +0200
Subject: [PATCH] Bug 1363922 - Part 2: Estimate scale of frames based on their
 PresShell resolution. r=mstange

MozReview-Commit-ID: 8f6T3PiPkF8
---
 layout/generic/nsGfxScrollFrame.cpp   | 10 +++++--
 layout/painting/FrameLayerBuilder.cpp | 40 ---------------------------
 layout/painting/FrameLayerBuilder.h   |  9 ------
 3 files changed, 7 insertions(+), 52 deletions(-)

diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index 80ddf51dbbd9..f49d517bfa58 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -2762,7 +2762,8 @@ ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange, nsIAtom* aOri
   nscoord appUnitsPerDevPixel = presContext->AppUnitsPerDevPixel();
   // 'scale' is our estimate of the scale factor that will be applied
   // when rendering the scrolled content to its own PaintedLayer.
-  gfxSize scale = FrameLayerBuilder::GetPaintedLayerScaleForFrame(mScrolledFrame);
+  float presShellResolution = mScrolledFrame->PresContext()->PresShell()->GetResolution();
+  gfxSize scale(presShellResolution, presShellResolution);
   nsPoint curPos = GetScrollPosition();
   nsPoint alignWithPos = mScrollPosForLayerPixelAlignment == nsPoint(-1,-1)
                          ? curPos : mScrollPosForLayerPixelAlignment;
@@ -5879,9 +5880,12 @@ ScrollFrameHelper::GetScrolledRect() const
   }
 
   // Now, snap the bottom right corner of both of these rects.
-  // We snap to layer pixels, so we need to respect the layer's scale.
+  // We ignore the CSS transforms here because we don't have a cheap way of accessing layer resolution of
+  // the layer that this frame is going to be painted to. Using the presShell resolution is good enough
+  // for most situations and simpler.
   nscoord appUnitsPerDevPixel = mScrolledFrame->PresContext()->AppUnitsPerDevPixel();
-  gfxSize scale = FrameLayerBuilder::GetPaintedLayerScaleForFrame(mScrolledFrame);
+  float presShellResolution = mScrolledFrame->PresContext()->PresShell()->GetResolution();
+  gfxSize scale(presShellResolution, presShellResolution);
   if (scale.IsEmpty()) {
     scale = gfxSize(1.0f, 1.0f);
   }
diff --git a/layout/painting/FrameLayerBuilder.cpp b/layout/painting/FrameLayerBuilder.cpp
index 33b6a3c10e3e..36303c074693 100644
--- a/layout/painting/FrameLayerBuilder.cpp
+++ b/layout/painting/FrameLayerBuilder.cpp
@@ -5875,46 +5875,6 @@ PredictScaleForContent(nsIFrame* aFrame, nsIFrame* aAncestorWithScale,
   return gfxSize(1.0, 1.0);
 }
 
-gfxSize
-FrameLayerBuilder::GetPaintedLayerScaleForFrame(nsIFrame* aFrame)
-{
-  MOZ_ASSERT(aFrame, "need a frame");
-  nsIFrame* last = nullptr;
-  for (nsIFrame* f = aFrame; f; f = nsLayoutUtils::GetCrossDocParentFrame(f)) {
-    last = f;
-
-    if (nsLayoutUtils::IsPopup(f)) {
-      // Don't examine ancestors of a popup. It won't make sense to check
-      // the transform from some content inside the popup to some content
-      // which is an ancestor of the popup.
-      break;
-    }
-
-    const SmallPointerArray<DisplayItemData>& array = aFrame->DisplayItemData();
-
-    for (uint32_t i = 0; i < array.Length(); i++) {
-      Layer* layer = DisplayItemData::AssertDisplayItemData(array.ElementAt(i))->mLayer;
-      ContainerLayer* container = layer->AsContainerLayer();
-      if (!container ||
-          !layer->Manager()->IsWidgetLayerManager()) {
-        continue;
-      }
-      for (Layer* l = container->GetFirstChild(); l; l = l->GetNextSibling()) {
-        PaintedDisplayItemLayerUserData* data =
-            static_cast<PaintedDisplayItemLayerUserData*>
-              (l->GetUserData(&gPaintedDisplayItemLayerUserData));
-        if (data) {
-          return PredictScaleForContent(aFrame, f, gfxSize(data->mXScale, data->mYScale));
-        }
-      }
-    }
-  }
-
-  float presShellResolution = last->PresContext()->PresShell()->GetResolution();
-  return PredictScaleForContent(aFrame, last,
-      gfxSize(presShellResolution, presShellResolution));
-}
-
 #ifdef MOZ_DUMP_PAINTING
 static void DebugPaintItem(DrawTarget& aDrawTarget,
                            nsPresContext* aPresContext,
diff --git a/layout/painting/FrameLayerBuilder.h b/layout/painting/FrameLayerBuilder.h
index 96133dc2d146..c8579f081855 100644
--- a/layout/painting/FrameLayerBuilder.h
+++ b/layout/painting/FrameLayerBuilder.h
@@ -568,15 +568,6 @@ public:
    */
   nsIntPoint GetLastPaintOffset(PaintedLayer* aLayer);
 
-  /**
-   * Return the resolution at which we expect to render aFrame's contents,
-   * assuming they are being painted to retained layers. This takes into account
-   * the resolution the contents of the ContainerLayer containing aFrame are
-   * being rendered at, as well as any currently-inactive transforms between
-   * aFrame and that container layer.
-   */
-  static gfxSize GetPaintedLayerScaleForFrame(nsIFrame* aFrame);
-
   /**
    * Stores a Layer as the dedicated layer in the DisplayItemData for a given frame/key pair.
    *