layout: add test reproducing a panic

src/layout.rs

@@ -960,6 +960,29 @@ mod test {
         assert_keys(&[], layout.keycodes());
     }
 
+    #[test]
+    fn stacked_hold_tap() {
+        static LAYERS: Layers<2, 1, 1> = [[[
+            k(Enter),
+            HoldTap(&HoldTapAction {
+                timeout: 10,
+                hold: k(LAlt),
+                tap: k(Space),
+                config: HoldTapConfig::Default,
+                tap_hold_interval: 0,
+            }),
+        ]]];
+
+        let mut layout = Layout::new(&LAYERS);
+        assert_eq!(CustomEvent::NoEvent, layout.tick());
+        // Push 2 events in a row, without ticking:
+        // Press/Release attached to a HoldTap
+        layout.event(Press(0, 1));
+        layout.event(Release(0, 1));
+        assert_eq!(CustomEvent::NoEvent, layout.tick());
+        assert_eq!(CustomEvent::NoEvent, layout.tick());
+    }
+
     #[test]
     fn multiple_actions() {
         static LAYERS: Layers<2, 1, 2> = [