Bug 1305957 part 13 - Highlight scroll anchor when 'layout.scroll-anchoring.highlight' is enabled. r=mattwoodrow

This commit adds a debug pref that will highlight the frames that are marked as
anchor nodes. This is helpful to debug anchor selection.

Differential Revision: https://phabricator.services.mozilla.com/D13276

--HG--
extra : rebase_source : 61109b3d967f0e4f8e5a260a71fecc01ebf22c73
extra : histedit_source : d2275fdb4edc3d7f6b006c8c7ab24fca1223998d

layout/generic/ScrollAnchorContainer.cpp

@@ -63,6 +63,14 @@ static void SetAnchorFlags(const nsIFrame* aScrolledFrame,
   }
   MOZ_ASSERT(frame,
              "The anchor node should be a descendant of the scroll frame");
+  // If needed, invalidate the frame so that we start/stop highlighting the
+  // anchor
+  if (StaticPrefs::layout_css_scroll_anchoring_highlight()) {
+    for (nsIFrame* frame = aAnchorNode->FirstContinuation(); !!frame;
+         frame = frame->GetNextContinuation()) {
+      frame->InvalidateFrame();
+    }
+  }
 }
 
 /**

layout/generic/nsFrame.cpp

@@ -22,6 +22,7 @@
 #include "mozilla/gfx/gfxVars.h"
 #include "mozilla/gfx/PathHelpers.h"
 #include "mozilla/Sprintf.h"
+#include "mozilla/StaticPrefs.h"
 
 #include "nsCOMPtr.h"
 #include "nsFlexContainerFrame.h"
@@ -3517,6 +3518,17 @@ void nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder* aBuilder,
       // Animations may change the stacking context state.
       !(child->MayHaveTransformAnimation() || child->MayHaveOpacityAnimation());
 
+  if (StaticPrefs::layout_css_scroll_anchoring_highlight()) {
+    if (child->FirstContinuation()->IsScrollAnchor()) {
+      nsRect bounds = child->GetContentRectRelativeToSelf() +
+                      aBuilder->ToReferenceFrame(child);
+      nsDisplaySolidColor* color = MakeDisplayItem<nsDisplaySolidColor>(
+          aBuilder, child, bounds, NS_RGBA(255, 0, 255, 64));
+      color->SetOverrideZIndex(INT32_MAX);
+      aLists.PositionedDescendants()->AppendToTop(color);
+    }
+  }
+
   if (doingShortcut) {
     BuildDisplayListForSimpleChild(aBuilder, child, aLists);
     return;

modules/libpref/init/StaticPrefList.h

@@ -896,6 +896,12 @@ VARCACHE_PREF(
 )
 #undef PREF_VALUE
 
+VARCACHE_PREF(
+  "layout.css.scroll-anchoring.highlight",
+   layout_css_scroll_anchoring_highlight,
+  bool, false
+)
+
 //---------------------------------------------------------------------------
 // JavaScript prefs
 //---------------------------------------------------------------------------