Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index 8ae2819e6a25525fb5c6aeba17bda724df869e7d..45fb553060e862cda2b80123bb61e2ba2348e329 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -238,16 +238,10 @@ void CompositingRequirementsUpdater::updateRecursive(PaintLayer* ancestorLayer,
 
         if (reasonsToComposite && layer->scrollsOverflow() && !layer->needsCompositedScrolling()) {
             // We will only set needsCompositedScrolling if we don't care about
-            // the LCD text hit, we may be able to switch to the compositor
+            // the LCD text hit, or when the background supports painting LCD text.
+            // We may be able to switch to the compositor
             // driven path if we're alread composited for other reasons and are
             // therefore using grayscale AA.
-            //
-            // FIXME: it should also be possible to promote if the layer can
-            // still use LCD text when promoted, but detecting when the
-            // compositor can do this is tricky. Currently, the layer must be
-            // both opaque and may only have an integer translation as its

[chrishtr, 2016/09/02 01:19:38]

Are you checking for integer translation?

[Stephen Chennney, 2016/09/06 21:06:21]

I could DCHECK for it. I believe we force integer scrolling if
needsCompositedScrolling is true, so we're OK.

[chrishtr, 2016/09/06 22:17:59]

The sum of the translations from the root must be integers to get LCD text. I
think this needs more investigation. Please file a bug. and add a TODO here.

[Stephen Chennney, 2016/09/07 19:57:54]

This comment on an earlier review explains why we have integer scroll offset:
https://codereview.chromium.org/1826013002/#msg55

I've added the bug number and a comment, but not here, rather in
CompositingReasonsFinder where it makes more sense. Here we are just adding
CompositingReasonOverflowScrollingTouch for cases where we have already given up
on getting LCD Text. The forthcoming patch reworks this comment to reflect what
is actually happening.

-            // transform. Both opacity and screen space transform are inherited
-            // properties, so this cannot be determined from local information.
             layer->getScrollableArea()->updateNeedsCompositedScrolling(PaintLayerScrollableArea::IgnoreLCDText);
             if (layer->needsCompositedScrolling())
                 reasonsToComposite |= CompositingReasonOverflowScrollingTouch;