Scrollbar ThumbLength is not updated when window size is changed.

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index c0b2a728e3c9077c46a7f194dce94174d0780fd0..9a057fa77ece362a24a7939823a62dd5a2ae3528 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -41,7 +41,8 @@ PaintedScrollbarLayer::PaintedScrollbarLayer(scoped_ptr<Scrollbar> scrollbar,
       thumb_thickness_(scrollbar_->ThumbThickness()),
       thumb_length_(scrollbar_->ThumbLength()),
       is_overlay_(scrollbar_->IsOverlay()),
-      has_thumb_(scrollbar_->HasThumb()) {
+      has_thumb_(scrollbar_->HasThumb()),
+      has_thumb_pre_(false) {

[Sikugu_, 2014/09/01 14:46:19]

can we have some better name here like cache_thumb_value_?

   if (!scrollbar_->IsOverlay())
     SetShouldScrollOnMainThread(true);
 }
@@ -189,9 +190,10 @@ void PaintedScrollbarLayer::UpdateThumbAndTrackGeometry() {
   UpdateProperty(scrollbar_->Location(), &location_);
   UpdateProperty(scrollbar_->IsOverlay(), &is_overlay_);
   UpdateProperty(scrollbar_->HasThumb(), &has_thumb_);
-  if (has_thumb_) {
+  if ((has_thumb_ != has_thumb_pre_) || has_thumb_) {

[danakj, 2014/09/02 16:23:06]

I don't understand, when has_thumb_ became false, you still want to update it?
But you don't have a thumb anymore.

[MuVen, 2014/09/02 16:42:04]

When "Pin it" is clicked a pop-up of window height of 367 and content height of
451 is created, during that time has_thumb_ is true, but later bec of some js
window height is increased to 451.
When window height is increased to 451 has_thumb_ is false, and thumb_length
should be zero. As has_thumb_ is false UpdateProperty on thumb_length_ is not
updated to 0 
(which is 202 previously).

[danakj, 2014/09/02 18:38:55]

Ok, then how about when has_thumb is false, we always UpdateProperty of these 2
things to 0?

Also can you please add unit tests for this.

[MuVen, 2014/09/03 07:04:39]

When has_thumb is false, UpdateProperty of ThumbLength() is calculated @
ScrollbarTheme::thumbLength().If ScrollBar is disabled Length will be 0. Test
Case is added.

     UpdateProperty(scrollbar_->ThumbThickness(), &thumb_thickness_);
     UpdateProperty(scrollbar_->ThumbLength(), &thumb_length_);
+    has_thumb_pre_ = has_thumb_;
   }
 }