Fix the wrong layout being used until window size changes

While looking into what had regressed https://github.com/element-hq/element-call/issues/3588, I found that 28047217b8 had filled in a couple of behaviors with non-reactive default values, the "natural window mode" behavior being among them. This meant that the app would no longer determine the correct window mode upon joining a call, instead always guessing "normal" as the value. This change restores its reactivity.

src/state/CallViewModel/CallViewModel.test.ts

@@ -502,6 +502,48 @@ describe("CallViewModel", () => {
     });
   });
 
+  test("layout reacts to window size", () => {
+    withTestScheduler(({ behavior, schedule, expectObservable }) => {
+      const windowSizeInputMarbles = "abc";
+      const expectedLayoutMarbles = " abc";
+      withCallViewModel(
+        {
+          remoteParticipants$: constant([aliceParticipant]),
+          rtcMembers$: constant([localRtcMember, aliceRtcMember]),
+          windowSize$: behavior(windowSizeInputMarbles, {
+            a: { width: 300, height: 600 }, // Start very narrow, like a phone
+            b: { width: 1000, height: 800 }, // Go to normal desktop window size
+            c: { width: 200, height: 180 }, // Go to PiP size
+          }),
+        },
+        (vm) => {
+          expectObservable(summarizeLayout$(vm.layout$)).toBe(
+            expectedLayoutMarbles,
+            {
+              a: {
+                // This is the expected one-on-one layout for a narrow window
+                type: "spotlight-expanded",
+                spotlight: [`${aliceId}:0`],
+                pip: `${localId}:0`,
+              },
+              b: {
+                // In a larger window, expect the normal one-on-one layout
+                type: "one-on-one",
+                local: `${localId}:0`,
+                remote: `${aliceId}:0`,
+              },
+              c: {
+                // In a PiP-sized window, we of course expect a PiP layout
+                type: "pip",
+                spotlight: [`${aliceId}:0`],
+              },
+            },
+          );
+        },
+      );
+    });
+  });
+
   test("spotlight speakers swap places", () => {
     withTestScheduler(({ behavior, schedule, expectObservable }) => {
       // Go immediately into spotlight mode for the test

src/state/CallViewModel/CallViewModel.ts

@@ -147,6 +147,8 @@ export interface CallViewModelOptions {
   livekitRoomFactory?: (options?: RoomOptions) => LivekitRoom;
   /** Optional behavior overriding the local connection state, mainly for testing purposes. */
   connectionState$?: Behavior<ConnectionState>;
+  /** Optional behavior overriding the computed window size, mainly for testing purposes. */
+  windowSize$?: Behavior<{ width: number; height: number }>;
 }
 
 // Do not play any sounds if the participant count has exceeded this
@@ -949,11 +951,19 @@ export function createCallViewModel$(
 
   const pipEnabled$ = scope.behavior(setPipEnabled$, false);
 
+  const windowSize$ =
+    options.windowSize$ ??
+    scope.behavior<{ width: number; height: number }>(
+      fromEvent(window, "resize").pipe(
+        startWith(null),
+        map(() => ({ width: window.innerWidth, height: window.innerHeight })),
+      ),
+    );
+
+  // A guess at what the window's mode should be based on its size and shape.
   const naturalWindowMode$ = scope.behavior<WindowMode>(
-    fromEvent(window, "resize").pipe(
+    windowSize$.pipe(
-      map(() => {
+      map(({ width, height }) => {
-        const height = window.innerHeight;
-        const width = window.innerWidth;
         if (height <= 400 && width <= 340) return "pip";
         // Our layouts for flat windows are better at adapting to a small width
         // than our layouts for narrow windows are at adapting to a small height,
@@ -963,7 +973,6 @@ export function createCallViewModel$(
         return "normal";
       }),
     ),
-    "normal",
   );
 
   /**

src/state/CallViewModel/CallViewModelTestUtils.ts

@@ -75,6 +75,7 @@ export interface CallViewModelInputs {
   speaking: Map<Participant, Observable<boolean>>;
   mediaDevices: MediaDevices;
   initialSyncState: SyncState;
+  windowSize$: Behavior<{ width: number; height: number }>;
 }
 
 const localParticipant = mockLocalParticipant({ identity: "" });
@@ -89,6 +90,7 @@ export function withCallViewModel(
     speaking = new Map(),
     mediaDevices = mockMediaDevices({}),
     initialSyncState = SyncState.Syncing,
+    windowSize$ = constant({ width: 1000, height: 800 }),
   }: Partial<CallViewModelInputs> = {},
   continuation: (
     vm: CallViewModel,
@@ -173,6 +175,7 @@ export function withCallViewModel(
           setE2EEEnabled: async () => Promise.resolve(),
         }),
       connectionState$,
+      windowSize$,
     },
     raisedHands$,
     reactions$,