Restore scrolloffset after exiting fullscreen.

android_webview/java/src/org/chromium/android_webview/AwContents.java

Index: android_webview/java/src/org/chromium/android_webview/AwContents.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContents.java b/android_webview/java/src/org/chromium/android_webview/AwContents.java
index 8ecc734cec45b2daa9e8f9a2957948b62baab280..fe1d4bf2f9a147641f22ec72c048881688ccd29b 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -323,13 +323,20 @@ public class AwContents implements SmartClipProvider,
     /**
      * A class that stores the state needed to enter and exit fullscreen.
      */
-    private static class FullScreenTransitionsState {
+    private class FullScreenTransitionsState {
         private final ViewGroup mInitialContainerView;
         private final InternalAccessDelegate mInitialInternalAccessAdapter;
         private final AwViewMethods mInitialAwViewMethods;
+        private float mInitialPageScaleFactor;
+        private int[] mInitialScrollOffset;
         private FullScreenView mFullScreenView;
         /** Whether the initial container view was focused when we entered fullscreen */
         private boolean mWasInitialContainerViewFocused;
+        /**
+         * Wheter the WebView needs to restore to the original scroll offset after
+         * exiting full screen
+         */
+        private boolean mNeedRestoreIntialScrollOffest;
 
         private FullScreenTransitionsState(ViewGroup initialContainerView,
                 InternalAccessDelegate initialInternalAccessAdapter,
@@ -343,6 +350,9 @@ public class AwContents implements SmartClipProvider,
                 boolean wasInitialContainerViewFocused) {
             mFullScreenView = fullScreenView;
             mWasInitialContainerViewFocused = wasInitialContainerViewFocused;
+            mInitialPageScaleFactor = AwContents.this.mPageScaleFactor;
+            mInitialScrollOffset =
+                    new int[] {mContainerView.getScrollX(), mContainerView.getScrollY()};
         }
 
         private boolean wasInitialContainerViewFocused() {
@@ -351,6 +361,15 @@ public class AwContents implements SmartClipProvider,
 
         private void exitFullScreen() {
             mFullScreenView = null;
+
+            // When the current (fullscreen) page scale factor is not the same as
+            // the initial page scale factor, WebView needs to restore the
+            // initial scroll offset when the page scale factor is restored to
+            // the initial value. This happens quite a few onDraws later after
+            // exitFullScreen(), when the page scale factor is propogated from
+            // blink to compositor thread.
+            mNeedRestoreIntialScrollOffest =
+                    (mInitialPageScaleFactor != AwContents.this.mPageScaleFactor);
         }
 
         private boolean isFullScreen() {
@@ -369,9 +388,31 @@ public class AwContents implements SmartClipProvider,
             return mInitialAwViewMethods;
         }
 
+        private float getInitialPageScaleFactor() {
+            return mInitialPageScaleFactor;
+        }
+
+        private int[] getInitialScrollOffset() {
+            return mInitialScrollOffset;
+        }
+
         private FullScreenView getFullScreenView() {
             return mFullScreenView;
         }
+
+        private void restoreScrollOffsetIfNeeded() {
+            if (mNeedRestoreIntialScrollOffest
+                    && (AwContents.this.mPageScaleFactor == mInitialPageScaleFactor)) {
+                mHandler.post(new Runnable() {
+                    @Override
+                    public void run() {
+                        AwContents.this.scrollContainerViewTo(
+                                mInitialScrollOffset[0], mInitialScrollOffset[1]);
+                        mNeedRestoreIntialScrollOffest = false;
+                    }
+                });
+            }
+        }
     }
 
     // Reference to the active mNativeAwContents pointer while it is active use
@@ -2793,6 +2834,8 @@ public class AwContents implements SmartClipProvider,
             mContentsClient.getCallbackHelper().postOnScaleChangedScaled(
                     (float) (oldPageScaleFactor * mDIPScale),
                     (float) (mPageScaleFactor * mDIPScale));
+
+            mFullScreenTransitionsState.restoreScrollOffsetIfNeeded();
         }
     }