Replace many usages of useObservableEagerState with useBehavior

This hook is simpler in its implementation (therefore hopefully more correct & performant) and enforces a type-level distinction between raw Observables and Behaviors.

src/grid/Grid.tsx

@@ -24,16 +24,16 @@ import {
   createContext,
   memo,
   use,
+  useCallback,
   useEffect,
   useMemo,
   useRef,
   useState,
+  useSyncExternalStore,
 } from "react";
 import useMeasure from "react-use-measure";
 import classNames from "classnames";
 import { logger } from "matrix-js-sdk/lib/logger";
-import { useObservableEagerState } from "observable-hooks";
-import { fromEvent, map, startWith } from "rxjs";
 
 import styles from "./Grid.module.css";
 import { useMergedRefs } from "../useMergedRefs";
@@ -155,11 +155,6 @@ export function useVisibleTiles(callback: VisibleTilesCallback): void {
   );
 }
 
-const windowHeightObservable$ = fromEvent(window, "resize").pipe(
-  startWith(null),
-  map(() => window.innerHeight),
-);
-
 export interface LayoutProps<LayoutModel, TileModel, R extends HTMLElement> {
   ref?: Ref<R>;
   model: LayoutModel;
@@ -261,7 +256,13 @@ export function Grid<
   const [gridRoot, gridRef2] = useState<HTMLElement | null>(null);
   const gridRef = useMergedRefs<HTMLElement>(gridRef1, gridRef2);
 
-  const windowHeight = useObservableEagerState(windowHeightObservable$);
+  const windowHeight = useSyncExternalStore(
+    useCallback((onChange) => {
+      window.addEventListener("resize", onChange);
+      return (): void => window.removeEventListener("resize", onChange);
+    }, []),
+    useCallback(() => window.innerHeight, []),
+  );
   const [layoutRoot, setLayoutRoot] = useState<HTMLElement | null>(null);
   const [generation, setGeneration] = useState<number | null>(null);
   const [visibleTilesCallback, setVisibleTilesCallback] =

src/grid/OneOnOneLayout.tsx

@@ -13,6 +13,7 @@ import { type OneOnOneLayout as OneOnOneLayoutModel } from "../state/CallViewMod
 import { type CallLayout, arrangeTiles } from "./CallLayout";
 import styles from "./OneOnOneLayout.module.css";
 import { type DragCallback, useUpdateLayout } from "./Grid";
+import { useBehavior } from "../useBehavior";
 
 /**
  * An implementation of the "one-on-one" layout, in which the remote participant
@@ -32,7 +33,7 @@ export const makeOneOnOneLayout: CallLayout<OneOnOneLayoutModel> = ({
   scrolling: function OneOnOneLayoutScrolling({ ref, model, Slot }): ReactNode {
     useUpdateLayout();
     const { width, height } = useObservableEagerState(minBounds$);
-    const pipAlignmentValue = useObservableEagerState(pipAlignment$);
+    const pipAlignmentValue = useBehavior(pipAlignment$);
     const { tileWidth, tileHeight } = useMemo(
       () => arrangeTiles(width, height, 1),
       [width, height],

src/grid/SpotlightExpandedLayout.tsx

@@ -6,12 +6,12 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type ReactNode, useCallback } from "react";
-import { useObservableEagerState } from "observable-hooks";
 
 import { type SpotlightExpandedLayout as SpotlightExpandedLayoutModel } from "../state/CallViewModel";
 import { type CallLayout } from "./CallLayout";
 import { type DragCallback, useUpdateLayout } from "./Grid";
 import styles from "./SpotlightExpandedLayout.module.css";
+import { useBehavior } from "../useBehavior";
 
 /**
  * An implementation of the "expanded spotlight" layout, in which the spotlight
@@ -46,7 +46,7 @@ export const makeSpotlightExpandedLayout: CallLayout<
     Slot,
   }): ReactNode {
     useUpdateLayout();
-    const pipAlignmentValue = useObservableEagerState(pipAlignment$);
+    const pipAlignmentValue = useBehavior(pipAlignment$);
 
     const onDragPip: DragCallback = useCallback(
       ({ xRatio, yRatio }) =>

src/grid/SpotlightPortraitLayout.tsx

@@ -13,6 +13,7 @@ import { type CallLayout, arrangeTiles } from "./CallLayout";
 import { type SpotlightPortraitLayout as SpotlightPortraitLayoutModel } from "../state/CallViewModel";
 import styles from "./SpotlightPortraitLayout.module.css";
 import { useUpdateLayout, useVisibleTiles } from "./Grid";
+import { useBehavior } from "../useBehavior";
 
 interface GridCSSProperties extends CSSProperties {
   "--grid-gap": string;
@@ -65,8 +66,7 @@ export const makeSpotlightPortraitLayout: CallLayout<
       width,
       model.grid.length,
     );
-    const withIndicators =
-      useObservableEagerState(model.spotlight.media$).length > 1;
+    const withIndicators = useBehavior(model.spotlight.media$).length > 1;
 
     return (
       <div