Draggable peeking/fullscreen launcher with transparent background.

ash/shelf/wm_shelf.h

Index: ash/shelf/wm_shelf.h
diff --git a/ash/shelf/wm_shelf.h b/ash/shelf/wm_shelf.h
index e943e8dee626018d8dee1f364eca80ff2fe49bca..800aaba186112abaed3cf457f91368ada79d7ee2 100644
--- a/ash/shelf/wm_shelf.h
+++ b/ash/shelf/wm_shelf.h
@@ -97,6 +97,10 @@ class ASH_EXPORT WmShelf : public ShelfLayoutManagerObserver {
 
   ShelfBackgroundType GetBackgroundType() const;
 
+  // Sets the shelf background to translucent while the app list is active in
+  // order to prevent overlapping opaque backgrounds.
+  void AppListIsActive(bool hideBackground);

[vadimt, 2017/05/22 23:36:56]

Should be called something like SetBackgroundOpacity?

[newcomer, 2017/05/25 23:10:51]

It just lets the ShelfLayoutManager know that the applist is active in order to
restrict the background behavior (so it doesn't strictly speaking set the
opacity). Updated comment! LMK if you still think it should be renamed.

+
   void UpdateVisibilityState();
 
   ShelfVisibilityState GetVisibilityState() const;
@@ -172,6 +176,11 @@ class ASH_EXPORT WmShelf : public ShelfLayoutManagerObserver {
   // TODO(mash): Facilitate simliar functionality in mash: crbug.com/636647
   std::unique_ptr<ShelfBezelEventHandler> bezel_event_handler_;
 
+  // Set with AppListIsActive, blocks SetPaintsBackground from painting a
+  // transparent background while the app list is active, which prevents the
+  // two transparent backgrounds from overlapping.
+  bool app_list_is_active_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(WmShelf);
 };