Prevent unrealistic climbing by modulating ground acceleration by ground plane horizontality

characters/base/character.gd

@@ -611,7 +611,13 @@ func _do_standard_motion(delta: float) -> void:
 				)
 			# Then, check which forward acceleration value to use.
 			var max_accel: float = (
-				_air_acceleration if use_air_physics else _ground_acceleration
+				_air_acceleration if use_air_physics else (
+					# If we're using ground acceleration,
+					# modulate it by the horizontality of the ground plane.
+					# This helps prevent unrealistic climbing.
+					_ground_acceleration *
+					abs(ground_normal.dot(Vector3.UP))
+				)
 			)
 			# Then, calculate true forward acceleration as follows:
 			# from standing at rest,