Consider any sufficiently short window 'flat'

This is because our layouts for flat windows are good at adapting to both small width and small height, while our layouts for narrow windows aren't so good at adapting to a small height.

src/state/CallViewModel.ts

@@ -526,8 +526,11 @@ export class CallViewModel extends ViewModel {
       const height = window.innerHeight;
       const width = window.innerWidth;
       if (height <= 400 && width <= 340) return "pip";
-      if (width <= 600) return "narrow";
+      // Our layouts for flat windows are better at adapting to a small width
+      // than our layouts for narrow windows are at adapting to a small height,
+      // so we give "flat" precedence here
       if (height <= 660) return "flat";
+      if (width <= 600) return "narrow";
       return "normal";
     }),
     distinctUntilChanged(),