python ui: implement inertial scrolling for GuiScrollPanel (#34596)

implement inertial scrolling for GuiScrollPanel

system/ui/lib/scroll_panel.py

@@ -2,6 +2,9 @@ import pyray as rl
 from cffi import FFI
 
 MOUSE_WHEEL_SCROLL_SPEED = 30
+INERTIA_FRICTION = 0.95  # The rate at which the inertia slows down
+MIN_VELOCITY = 0.1  # Minimum velocity before stopping the inertia
+
 
 class GuiScrollPanel:
   def __init__(self, bounds: rl.Rectangle, content: rl.Rectangle, show_vertical_scroll_bar: bool = False):
@@ -12,22 +15,28 @@ class GuiScrollPanel:
     self._scroll = rl.Vector2(0, 0)
     self._view = rl.Rectangle(0, 0, 0, 0)
     self._show_vertical_scroll_bar: bool = show_vertical_scroll_bar
+    self._velocity_y = 0.0  # Velocity for inertia
 
-  def handle_scroll(self)-> rl.Vector2:
+  def handle_scroll(self) -> rl.Vector2:
     mouse_pos = rl.get_mouse_position()
+
+    # Handle dragging logic
     if rl.check_collision_point_rec(mouse_pos, self._bounds) and rl.is_mouse_button_pressed(rl.MouseButton.MOUSE_BUTTON_LEFT):
       if not self._dragging:
         self._dragging = True
         self._last_mouse_y = mouse_pos.y
+        self._velocity_y = 0.0  # Reset velocity when drag starts
 
     if self._dragging:
       if rl.is_mouse_button_down(rl.MouseButton.MOUSE_BUTTON_LEFT):
         delta_y = mouse_pos.y - self._last_mouse_y
         self._scroll.y += delta_y
         self._last_mouse_y = mouse_pos.y
+        self._velocity_y = delta_y  # Update velocity during drag
       else:
         self._dragging = False
 
+    # Handle mouse wheel scrolling
     wheel_move = rl.get_mouse_wheel_move()
     if self._show_vertical_scroll_bar:
       self._scroll.y += wheel_move * (MOUSE_WHEEL_SCROLL_SPEED - 20)
@@ -37,4 +46,17 @@ class GuiScrollPanel:
       max_scroll_y = self._content.height - self._bounds.height
       self._scroll.y = max(min(self._scroll.y, 0), -max_scroll_y)
 
+    # Apply inertia (continue scrolling after mouse release)
+    if not self._dragging:
+      self._scroll.y += self._velocity_y
+      self._velocity_y *= INERTIA_FRICTION  # Slow down velocity over time
+
+      # Stop scrolling when velocity is low
+      if abs(self._velocity_y) < MIN_VELOCITY:
+        self._velocity_y = 0.0
+
+    # Ensure scrolling doesn't go beyond bounds
+    max_scroll_y = max(self._content.height - self._bounds.height, 0)
+    self._scroll.y = max(min(self._scroll.y, 0), -max_scroll_y)
+
     return self._scroll