Factor out a layout switch view + view model

So it can easily be shown and hidden wholesale.

src/state/LayoutSwitchViewModel.ts

@@ -0,0 +1,99 @@
+/*
+Copyright 2025 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  combineLatest,
+  map,
+  Subject,
+  startWith,
+  skipWhile,
+  switchAll,
+} from "rxjs";
+
+import { type WindowMode } from "./CallViewModel/CallViewModel.ts";
+import { constant, type Behavior } from "./Behavior.ts";
+import { type ObservableScope } from "./ObservableScope.ts";
+
+export type LayoutMode = "spotlight" | "grid";
+
+export interface LayoutSwitchViewModel {
+  /**
+   * The layout mode of the call's media tiles.
+   */
+  layout$: Behavior<LayoutMode>;
+  setLayout: (value: LayoutMode) => void;
+}
+
+/**
+ * Creates a layout mode switch that allows switching between grid and spotlight layouts.
+ * The actual layout mode might switch automatically to spotlight if there is a
+ * remote screen share active or if the window mode is flat.
+ *
+ * @param scope - The observable scope to manage subscriptions.
+ * @param windowMode$ - The current window mode.
+ * @param hasRemoteScreenShares$ - A behavior indicating if there are remote screen shares active.
+ */
+export function createLayoutSwitchViewModel(
+  scope: ObservableScope,
+  windowMode$: Behavior<WindowMode>,
+  hasRemoteScreenShares$: Behavior<boolean>,
+): LayoutSwitchViewModel {
+  const userSelection$ = new Subject<LayoutMode>();
+  // Callback to set the layout desired by the user.
+  // Notice that this is only a preference, the actual layout can be overridden
+  // if there is a remote screen share active.
+  const setLayout = (value: LayoutMode): void => userSelection$.next(value);
+
+  /**
+   * The natural layout - the layout that the interface would prefer to be in,
+   * not accounting for the user's manual selections.
+   */
+  const naturalLayout$ = scope.behavior<LayoutMode>(
+    combineLatest(
+      [hasRemoteScreenShares$, windowMode$],
+      (hasRemoteScreenShares, windowMode) => {
+        // When the window is flat (as with a phone in landscape orientation),
+        // grid is preferable as there's usually more than enough horizontal
+        // space to fit in some grid tiles on the side.
+        if (windowMode === "flat") return "grid";
+        // When there are screen shares, spotlight is a better experience. We
+        // want them to be big and readable.
+        return hasRemoteScreenShares ? "spotlight" : "grid";
+      },
+    ),
+  );
+
+  /**
+   * The layout mode of the call's media tiles.
+   */
+  const layout$ = scope.behavior<LayoutMode>(
+    // Whenever the user makes a selection, we enter a new mode of behavior:
+    userSelection$.pipe(
+      map((selection) => {
+        if (selection === "grid")
+          // The user has selected grid. Start by respecting their choice, but
+          // then follow the natural mode again as soon as it matches.
+          return naturalLayout$.pipe(
+            skipWhile((naturalMode) => naturalMode !== selection),
+            startWith(selection),
+          );
+
+        // The user has selected spotlight. If this matches the natural layout,
+        // then follow the natural layout going forward.
+        return selection === naturalLayout$.value
+          ? naturalLayout$
+          : constant(selection);
+      }),
+      // Initially the mode of behavior is to just follow the natural layout.
+      startWith(naturalLayout$),
+      // Switch between each mode of behavior.
+      switchAll(),
+    ),
+  );
+
+  return { layout$, setLayout };
+}