Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index 0cc3aa168cc781f4b65e8058ebfa727a2c691ea6..fe31bb28925a3d77f3fa771e98ed977210022d84 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -544,8 +544,11 @@ void Scrollbar::SetEnabled(bool e) {
   enabled_ = e;
   GetTheme().UpdateEnabledState(*this);
 
-  ScrollbarPart invalid_parts = GetTheme().InvalidateOnEnabledChange();
-  SetNeedsPaintInvalidation(invalid_parts);
+  // We can skip thumb/track repaint when hiding an overlay scrollbar, but not
+  // when showing (since the proportions may have changed while hidden).
+  bool skipPartsRepaint = IsOverlayScrollbar() && scrollable_area_ &&
+                          scrollable_area_->ScrollbarsHidden();
+  SetNeedsPaintInvalidation(skipPartsRepaint ? kNoPart : kAllParts);

[bokan, 2017/04/25 23:21:17]

On the CC side (for Aura anyway) we only need to paint once since we draw a 9
patch resource. This will cause us to upload a new texture to CC everytime we
fade in, right? Though, maybe that's ok for now - I can't think of a way to keep
the main thread and impl sides working in a clean way.

[skobes, 2017/04/26 01:28:58]

I think ideally we would not repurpose the enabled state for hiding.  The
problem is that painting while hidden stores a blank thumb resource, since
ScrollbarThemeOverlay::PaintThumb passes kStateDisabled.

If we made the part-repaints from cc ignore the hidden state (i.e. paint
resource as if visible) we could probably avoid the invalidations here and in
SetProportion.

[bokan, 2017/04/26 11:51:09]

It's an unfortunate quirk of main-thread painting that it reuses enabled state
for hiding/showing overlays. The CC side hides using opacity without requiring a
repaint but if the main thread invalidates it we'll repaint on the next update.
Unfortunately, we have to disable the scrollbar from CC while its hidden so this
will cause us to repaint on fade in. 

 }
 
 int Scrollbar::ScrollbarThickness() const {