fix(scroll): restart the stats window when an idle gap is dropped by size

#582 dropped an idle gap from the frame stats two ways: the reset_scroll()
sentinel, which also restarts the 5s window timer, and a size guard for
scrollers that never call reset_scroll(), which did not. On that path the
first real frame after the gap found the boundary overdue and logged a
stats line for a one-frame window. Both paths now share one seeding helper.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_scroll_helper.py

@@ -460,6 +460,19 @@ class TestIdleGapIsNotAFrame:
         assert not info.called
         assert len(helper._window) == 1
 
+    def test_a_dropped_gap_restarts_the_window_timer_too(self, helper):
+        """The size-guard path is the one scrollers that never call
+        reset_scroll() take, so it must restart the window like the sentinel
+        does, or their next scroll opens with a one-frame stats line."""
+        helper.log_frame_rate()                     # seeds
+        helper.last_frame_time = time.time() - 137.0
+        helper.last_fps_log_time = 0.0              # boundary long overdue
+        with patch.object(helper.logger, "info") as info:
+            helper.log_frame_rate()                 # the gap, dropped
+            helper.log_frame_rate()                 # first real frame
+        assert not info.called, "a one-frame window was reported"
+        assert len(helper._window) == 1
+
     def test_a_normal_frame_still_counts(self, helper):
         helper.last_frame_time = time.time() - 0.010
         helper.log_frame_rate()