scroll: use iOS-style weighted velocity averaging for fling (#37659)

* scroll: use iOS-style weighted velocity averaging for fling

Weight older velocity samples more heavily on finger release to produce
more consistent fling velocities. The last touch samples before lift are
noisy (finger decelerating, rotating, jittering), so we trust the earlier
steadier samples more: 60% oldest, 35% middle, 5% newest.

Reverse-engineered from iOS UIScrollView by the Flutter team.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Update system/ui/lib/application.py

* Apply suggestions from code review

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

system/ui/lib/application.py

@@ -169,6 +169,10 @@ class MouseState:
       self._rk.keep_time()
 
   def _handle_mouse_event(self):
+    # TODO: read touch events from evdev directly to get real kernel timestamps.
+    #  Polling at 140Hz with time.monotonic() causes timing jitter that makes scroll
+    #  velocity oscillate (alternating high/low). Real timestamps would also let us
+    #  detect swipe-stop-lift via event gaps instead of the fragile decel heuristic.
     for slot in range(MAX_TOUCH_SLOTS):
       mouse_pos = rl.get_touch_position(slot)
       x = mouse_pos.x / self._scale if self._scale != 1.0 else mouse_pos.x

system/ui/lib/scroll_panel2.py

@@ -20,6 +20,21 @@ MAX_SPEED = 10000.0  # px/s
 DEBUG = os.getenv("DEBUG_SCROLL", "0") == "1"
 
 
+# Weights older (steadier) velocity samples more heavily on release.
+# Finger-lift samples are noisy; trusting earlier samples gives consistent fling velocity.
+# Reverse-engineered from iOS UIScrollView (tuned at 120Hz touch) by Flutter team:
+# https://github.com/flutter/flutter/pull/60501
+# 3 samples ≈ 25ms at 120Hz (iOS) / ~21ms at 140Hz (comma). Scale if touch rate changes.
+def weighted_velocity(buffer: deque) -> float:
+  if len(buffer) >= 3:
+    return buffer[-3] * 0.6 + buffer[-2] * 0.35 + buffer[-1] * 0.05
+  elif len(buffer) == 2:
+    return buffer[-2] * 0.7 + buffer[-1] * 0.3
+  elif len(buffer) == 1:
+    return buffer[-1]
+  return 0.0
+
+
 # from https://ariya.io/2011/10/flick-list-with-its-momentum-scrolling-and-deceleration
 class ScrollState(Enum):
   STEADY = 0
@@ -151,7 +166,13 @@ class GuiScrollPanel2:
         # Touch rejection: when releasing finger after swiping and stopping, panel
         # reports a few erroneous touch events with high velocity, try to ignore.
 
-        # If velocity decelerates very quickly, assume user doesn't intend to auto scroll
+        # If velocity decelerates very quickly, assume user doesn't intend to auto scroll.
+        # Catches two cases: 1) swipe, stop finger, then lift (stale high velocity in buffer)
+        # 2) dirty finger lift where finger rotates/slides producing spurious velocity spike.
+        # TODO: this heuristic false-positives on fast swipes because 140Hz touch polling
+        #  jitter causes velocity to oscillate (not real deceleration). Better approaches:
+        #  - Use evdev kernel timestamps to eliminate velocity oscillation at the source
+        #  - Replace with a time-since-last-event check (40ms timeout) for swipe-stop-lift
         high_decel = False
         if len(self._velocity_buffer) > 2:
           # We limit max to first half since final few velocities can surpass first few
@@ -166,6 +187,8 @@ class GuiScrollPanel2:
               print('deceleration too high, going to STEADY')
             high_decel = True
 
+        self._velocity = weighted_velocity(self._velocity_buffer)
+
         # If final velocity is below some threshold, switch to steady state too
         low_speed = abs(self._velocity) <= MIN_VELOCITY_FOR_CLICKING * 1.5  # plus some margin