Scroll bars: the screen's GUI size, and a test of the drag

stats_scroll and stats_drag take the GUI height from gui_size() instead
of a copy of it. The Game Rules list's press, drag and release get the
test from #1913 (vlesmentie's take on the same fix).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/horizon-client/src/game/game_rules_screen.rs

@@ -564,6 +564,29 @@ mod tests {
         );
     }
 
+    /// AbstractScrollArea: a press on the bar holds it, a drag moves the
+    /// list (to the ends past them), a release lets go.
+    #[test]
+    fn the_scroll_bar_can_be_held_dragged_and_let_go() {
+        let mut s = RulesScreen::new(&GameRules::default());
+        let (w, h) = (427.0, 240.0);
+        let (top, bottom, content) = (HEADER, h - FOOTER, s.content());
+        assert!(
+            content > bottom - top,
+            "the rules overflow a 240-high screen"
+        );
+        let bar = RulesScreen::bar(w);
+        assert!(!s.press_scrollbar((bar - 4.0, top + 5.0), (w, h)));
+        assert!(s.press_scrollbar((bar + 3.0, top + 5.0), (w, h)));
+        s.drag(top + 40.0, h);
+        assert!(s.scroll.amount > 0.0);
+        s.drag(bottom + 10.0, h);
+        assert_eq!(s.scroll.amount, content - (bottom - top));
+        s.release();
+        s.drag(top, h);
+        assert_eq!(s.scroll.amount, content - (bottom - top));
+    }
+
     #[test]
     fn edits_change_rules_and_bad_numbers_hold_done() {
         let mut s = RulesScreen::new(&GameRules::default());

crates/horizon-client/src/game/stats_screen.rs

@@ -492,11 +492,7 @@ impl Game<'_> {
     }
     pub(super) fn stats_scroll(&mut self, amount: f32) {
         let (row_h, _) = self.stats_rows();
-        let size = self
-            .renderer
-            .as_ref()
-            .map_or([1280, 720], |r| [r.config.width, r.config.height]);
-        let h = HudBatch::new(size).height;
+        let h = self.gui_size().1;
         let content = self.stats_content();
         self.stats_ui
             .scroll
@@ -517,11 +513,7 @@ impl Game<'_> {
         if !self.stats_ui.scroll.held() {
             return;
         }
-        let size = self
-            .renderer
-            .as_ref()
-            .map_or([1280, 720], |r| [r.config.width, r.config.height]);
-        let h = HudBatch::new(size).height;
+        let h = self.gui_size().1;
         let content = self.stats_content();
         self.stats_ui
             .scroll