Fix new back arrow's "over eager" appearance

There are two fixes in this cl:
1. The back arrow launches into its ENTRY state too easily: guard it
   with a minimum motion threshold.
2. The cancel animation is triggered upon MotionEvent.ACITON_UP, even
   when there has not been sufficient motion to trigger a change from
   GONE -> ENTRY (invisible to appearance): prevent GONE -> CANCELLED
   state transitions.

Bug: 238907408
Test: manual
Change-Id: I5ab8042fc675cfc527b837578b1d415d2d061d79

packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt

diff --git a/packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt b/packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt
index a833670..28ab83c 100644
--- a/packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt
+++ b/packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt
@@ -30,6 +30,7 @@
 import android.view.MotionEvent
 import android.view.VelocityTracker
 import android.view.View
+import android.view.ViewConfiguration
 import android.view.WindowManager
 import android.view.animation.DecelerateInterpolator
 import android.view.animation.PathInterpolator
@@ -98,6 +99,7 @@
     context: Context,
     private var backAnimation: BackAnimation?,
     private val windowManager: WindowManager,
+    private val viewConfiguration: ViewConfiguration,
     @Main private val mainHandler: Handler,
     private val vibratorHelper: VibratorHelper,
     private val configurationController: ConfigurationController,
@@ -112,6 +114,7 @@
      */
     class Factory @Inject constructor(
         private val windowManager: WindowManager,
+        private val viewConfiguration: ViewConfiguration,
         @Main private val mainHandler: Handler,
         private val vibratorHelper: VibratorHelper,
         private val configurationController: ConfigurationController,
@@ -123,6 +126,7 @@
                 context,
                 backAnimation,
                 windowManager,
+                viewConfiguration,
                 mainHandler,
                 vibratorHelper,
                 configurationController,
@@ -164,6 +168,10 @@
 
     private var gestureStartTime = 0L
 
+    // Whether the current gesture has moved a sufficiently large amount,
+    // so that we can unambiguously start showing the ENTRY animation
+    private var hasPassedDragSlop = false
+
     private val failsafeRunnable = Runnable { onFailsafe() }
 
     private enum class GestureState {
@@ -304,18 +312,17 @@
                 startX = event.x
                 startY = event.y
                 gestureStartTime = SystemClock.uptimeMillis()
-
-                // Reset the arrow to the side
-                updateArrowState(GestureState.ENTRY)
-
-                windowManager.updateViewLayout(mView, layoutParams)
-                mView.startTrackingShowBackArrowLatency()
             }
-            MotionEvent.ACTION_MOVE -> handleMoveEvent(event)
+            MotionEvent.ACTION_MOVE -> {
+                // only go to the ENTRY state after some minimum motion has occurred
+                if (dragSlopExceeded(event.x, startX)) {
+                    handleMoveEvent(event)
+                }
+            }
             MotionEvent.ACTION_UP -> {
                 if (currentState == GestureState.ACTIVE) {
                     updateArrowState(if (isFlung()) GestureState.FLUNG else GestureState.COMMITTED)
-                } else {
+                } else if (currentState != GestureState.GONE) { // if invisible, skip animation
                     updateArrowState(GestureState.CANCELLED)
                 }
                 velocityTracker = null
@@ -330,6 +337,28 @@
         }
     }
 
+    /**
+     * Returns false until the current gesture exceeds the touch slop threshold,
+     * and returns true thereafter (we reset on the subsequent back gesture).
+     * The moment it switches from false -> true is important,
+     * because that's when we switch state, from GONE -> ENTRY.
+     * @return whether the current gesture has moved past a minimum threshold.
+     */
+    private fun dragSlopExceeded(curX: Float, startX: Float): Boolean {
+        if (hasPassedDragSlop) return true
+
+        if (abs(curX - startX) > viewConfiguration.scaledTouchSlop) {
+            // Reset the arrow to the side
+            updateArrowState(GestureState.ENTRY)
+
+            windowManager.updateViewLayout(mView, layoutParams)
+            mView.startTrackingShowBackArrowLatency()
+
+            hasPassedDragSlop = true
+        }
+        return hasPassedDragSlop
+    }
+
     private fun updateArrowStateOnMove(yTranslation: Float, xTranslation: Float) {
         if (!currentState.isInteractive())
             return
@@ -533,6 +562,7 @@
     }
 
     private fun resetOnDown() {
+        hasPassedDragSlop = false
         hasHapticPlayed = false
         totalTouchDelta = 0f
         vibrationTime = 0