Bug 1292034 - Make the StackScroller flywheel detection code more robust to real user behaviour when flinging. r=rbarker

MozReview-Commit-ID: 6OBCkd9bwRt

mobile/android/geckoview/src/main/java/org/mozilla/gecko/gfx/StackScroller.java

@@ -261,9 +261,14 @@ public class StackScroller {
         if (mFlywheel && !isFinished()) {
             float oldVelocityX = mScrollerX.mCurrVelocity;
             float oldVelocityY = mScrollerY.mCurrVelocity;
-            if (Math.signum(velocityX) == Math.signum(oldVelocityX)
-                    && Math.signum(velocityY) == Math.signum(oldVelocityY)) {
+            boolean sameXDirection = (velocityX == 0) || (oldVelocityX == 0) ||
+                ((velocityX < 0) == (oldVelocityX < 0));
+            boolean sameYDirection = (velocityY == 0) || (oldVelocityY == 0) ||
+                ((velocityY < 0) == (oldVelocityY < 0));
+            if (sameXDirection) {
                 velocityX += oldVelocityX;
+            }
+            if (sameYDirection) {
                 velocityY += oldVelocityY;
             }
         }