Pause scroll listener when GTS hides, resume when shown.

R=yusufo@chromium.org

Bug: 976213
Change-Id: Ia31e1a8c93c77e657bc7f0ca1b22d5bebe364e5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663383
Commit-Queue: Matt Simmons <mattsimmons@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670259}

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/TabListRecyclerView.java

@@ -76,8 +76,26 @@ class TabListRecyclerView extends RecyclerView {
     }
 
     private class TabListOnScrollListener extends RecyclerView.OnScrollListener {
+        // TODO(mattsimmons): Remove state from this class. This is here to prevent scroll signals
+        //  from showing the toolbar shadow after the show/hide of the GTS has already triggered the
+        //  shadow being hidden. Due to this view's visibility being updated asynchronously after
+        //  animating off-screen, checking View.getVisibility() can't be used as the guard condition
+        //  here. Removing/Adding the listener from the view also happens asynchronously and has
+        //  similar undesirable effects.
+        private boolean mIsActive;
+
+        public void pause() {
+            mIsActive = false;
+        }
+
+        public void resume() {
+            mIsActive = true;
+        }
+
         @Override
         public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
+            if (!mIsActive) return;
+
             final int yOffset = recyclerView.computeVerticalScrollOffset();
             setShadowVisibility(yOffset > 0);
         }
@@ -145,6 +163,7 @@ class TabListRecyclerView extends RecyclerView {
                 mListener.finishedShowing();
                 // Restore the original value.
                 setItemAnimator(mOriginalAnimator);
+                mScrollListener.resume();
                 setShadowVisibility(computeVerticalScrollOffset() > 0);
                 if (mDynamicView != null) {
                     mDynamicView.dropCachedBitmap();
@@ -302,6 +321,7 @@ class TabListRecyclerView extends RecyclerView {
                 mListener.finishedHiding();
             }
         });
+        mScrollListener.pause();
         setShadowVisibility(false);
         mFadeOutAnimator.start();
         if (!animate) mFadeOutAnimator.end();