perf(app): stop per-frame style recalculation in skeletons and member list (#1681)

fluxer_app/src/app/App.tsx

@@ -32,6 +32,7 @@ import {startDesktopLocaleBridge} from '@app/features/platform/utils/DesktopLoca
 import {PremiumCheckoutReturnWatcher} from '@app/features/premium/components/PremiumCheckoutReturnWatcher';
 import {QUICK_SWITCHER_PORTAL_ID} from '@app/features/search/components/quick_switcher/QuickSwitcherConstants';
 import {useCustomThemeStyle} from '@app/features/theme/hooks/useCustomThemeStyle';
+import {useRemScaleTracking} from '@app/features/theme/hooks/useRemScaleTracking';
 import {useThemeCssVariables} from '@app/features/theme/hooks/useThemeCssVariables';
 import Theme from '@app/features/theme/state/Theme';
 import ThemeLibrary from '@app/features/theme/state/ThemeLibrary';
@@ -169,6 +170,7 @@ export const AppWrapper = observer(({children}: AppWrapperProps) => {
 	}, [isNative, firstClickPassThroughWhenUnfocused]);
 	useDesktopAllowTransparency(isNative);
 	useWindowEventListeners({preventDocumentScroll: !isNative});
+	useRemScaleTracking();
 	usePlatformClasses(platform, isNative);
 	useThemeCssVariables({
 		effectiveTheme: Theme.effectiveTheme,

fluxer_app/src/features/app/components/skeleton/GuildRailSkeleton.tsx

@@ -24,6 +24,7 @@ import {
 	CompassIcon,
 	DownloadSimpleIcon,
 	type Icon,
+	type IconWeight,
 	PlusIcon,
 	QuestionMarkIcon,
 	StarIcon,
@@ -118,7 +119,15 @@ const InlineDMPlaceholder = ({selected, unread}: {readonly selected: boolean; re
 	</flx-app-guild-rail-skeleton-item>
 );
 
-const RailActionButton = ({icon: Icon, selected = false}: {readonly icon: Icon; readonly selected?: boolean}) => (
+const RailActionButton = ({
+	icon: Icon,
+	selected = false,
+	weight = 'bold',
+}: {
+	readonly icon: Icon;
+	readonly selected?: boolean;
+	readonly weight?: IconWeight;
+}) => (
 	<flx-app-guild-rail-skeleton-item
 		className={flxElementClassName(styles.item)}
 		data-selected={selectedAttribute(selected)}
@@ -129,7 +138,7 @@ const RailActionButton = ({icon: Icon, selected = false}: {readonly icon: Icon;
 			data-flx="app.skeleton.guild-rail-skeleton.rail-action-button.action-icon"
 		>
 			<Icon
-				weight="bold"
+				weight={weight}
 				className={styles.actionIconGlyph}
 				data-flx="app.skeleton.guild-rail-skeleton.rail-action-button.icon"
 			/>
@@ -578,6 +587,7 @@ export const GuildRailSkeleton: React.FC<GuildRailSkeletonProps> = ({
 						{discoveryVisible && (
 							<RailActionButton
 								icon={CompassIcon}
+								weight="fill"
 								selected={isDiscoverySelected}
 								data-flx="app.skeleton.guild-rail-skeleton.rail-action-button"
 							/>

fluxer_app/src/features/app/components/skeleton/Skeleton.module.css

@@ -1,11 +1,5 @@
 /* SPDX-License-Identifier: AGPL-3.0-or-later */
 
-@property --skeleton-shimmer-position {
-	syntax: '<percentage>';
-	inherits: true;
-	initial-value: -150%;
-}
-
 .skeleton {
 	--skeleton-opacity: var(--skeleton-opacity-default, 0.45);
 	--skeleton-width: 100%;
@@ -13,13 +7,13 @@
 	background-color: var(--background-modifier-accent);
 	background-image: linear-gradient(
 		105deg,
-		transparent 46%,
+		transparent 0%,
 		color-mix(in srgb, var(--text-primary) 7%, transparent) 50%,
-		transparent 54%
+		transparent 100%
 	);
-	background-size: 200vw 100%;
+	background-size: 200% 100%;
-	background-position: var(--skeleton-shimmer-position) 0;
+	background-position: -50% 0;
-	background-attachment: fixed;
+	animation: skeleton-shimmer 3.2s linear infinite;
 	opacity: var(--skeleton-opacity);
 	pointer-events: none;
 	user-select: none;
@@ -30,35 +24,25 @@
 	max-width: 100%;
 }
 
-:global(html):has(.skeleton) {
-	animation: skeleton-shimmer 3.2s ease-in-out infinite;
-}
-
 @keyframes skeleton-shimmer {
 	from {
-		--skeleton-shimmer-position: -150%;
+		background-position: -50% 0;
 	}
 
 	to {
-		--skeleton-shimmer-position: 150%;
+		background-position: 150% 0;
 	}
 }
 
 @media (prefers-reduced-motion: reduce) {
-	:global(html):has(.skeleton) {
-		animation: none;
-	}
-
 	.skeleton {
+		animation: none;
 		background-image: none;
 	}
 }
 
-:global(html.reduced-motion):has(.skeleton) {
-	animation: none;
-}
-
 :global(html.reduced-motion) .skeleton {
+	animation: none;
 	background-image: none;
 }
 

fluxer_app/src/features/app/hooks/usePlatformClasses.ts

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
+import {clearRemScaleCache} from '@app/features/theme/layout/RemFromPx';
 import type {NativePlatform} from '@app/features/ui/utils/NativeUtils';
 import {useLayoutEffect} from 'react';
 
@@ -8,8 +9,10 @@ export function usePlatformClasses(platform: NativePlatform, isNative: boolean):
 		const htmlNode = document.documentElement;
 		const platformClasses = [isNative ? 'platform-native' : 'platform-web', `platform-${platform}`];
 		htmlNode.classList.add(...platformClasses);
+		clearRemScaleCache();
 		return () => {
 			htmlNode.classList.remove(...platformClasses);
+			clearRemScaleCache();
 		};
 	}, [isNative, platform]);
 }

fluxer_app/src/features/channel/components/ChannelMembers.tsx

@@ -95,6 +95,41 @@ function measureMemberGroupHeadingWidthPx(element: HTMLElement | null): number {
 	return Math.round((right - left) / getRemScaleForDocument(element.ownerDocument));
 }
 
+function createGroupHeadingRegistrar(groupHeadingWidths: Map<string, number>): (node: HTMLDivElement | null) => void {
+	return (node) => {
+		if (node == null) {
+			return;
+		}
+		const groupId = node.dataset.memberGroupId;
+		if (groupId == null) {
+			return;
+		}
+		const widthPx = measureMemberGroupHeadingWidthPx(node);
+		if (widthPx !== SKELETON_UNMEASURED_WIDTH_PX) {
+			groupHeadingWidths.set(groupId, widthPx);
+		}
+	};
+}
+
+interface GroupHeadingWidthTracking {
+	contentKey: string;
+	groupHeadingWidths: Map<string, number>;
+	registerGroupHeading: (node: HTMLDivElement | null) => void;
+}
+
+function useGroupHeadingWidthTracking(contentKey: string): GroupHeadingWidthTracking {
+	const trackingRef = useRef<GroupHeadingWidthTracking | null>(null);
+	if (trackingRef.current == null || trackingRef.current.contentKey !== contentKey) {
+		const groupHeadingWidths = trackingRef.current?.groupHeadingWidths ?? new Map<string, number>();
+		trackingRef.current = {
+			contentKey,
+			groupHeadingWidths,
+			registerGroupHeading: createGroupHeadingRegistrar(groupHeadingWidths),
+		};
+	}
+	return trackingRef.current;
+}
+
 interface MemberListGroupHeaderContentProps {
 	name: string;
 	count: number;
@@ -129,13 +164,14 @@ interface GroupDMMemberListGroupProps {
 	group: GroupDMMemberGroup;
 	channelId: string;
 	ownerId: string | null;
-	onHeadingRef: (groupId: string, node: HTMLDivElement | null) => void;
+	onHeadingRef: (node: HTMLDivElement | null) => void;
 }
 
 const GroupDMMemberListGroup = observer(({group, channelId, ownerId, onHeadingRef}: GroupDMMemberListGroupProps) => (
 	<div className={styles.groupContainer} data-flx="channel.channel-members.group-dm-member-list-group.group-container">
 		<div
-			ref={(node) => onHeadingRef(group.id, node)}
+			ref={onHeadingRef}
+			data-member-group-id={group.id}
 			className={styles.groupHeader}
 			data-flx="channel.channel-members.group-dm-member-list-group.group-header"
 		>
@@ -196,13 +232,7 @@ const LazyMemberList = observer(function LazyMemberList({guild, channel}: LazyMe
 	const groups = memberListState?.groups ?? [];
 	const populatedGroups = useMemo(() => groups.filter((group) => group.count > 0), [groups]);
 	const memberGroupCountsKey = populatedGroups.map((group) => `${group.id}:${group.count}`).join(',');
-	const groupHeadingWidthsRef = useRef(new Map<string, number>());
+	const {groupHeadingWidths, registerGroupHeading} = useGroupHeadingWidthTracking(memberGroupCountsKey);
-	const registerGroupHeading = useCallback((groupId: string, node: HTMLDivElement | null) => {
-		const widthPx = measureMemberGroupHeadingWidthPx(node);
-		if (widthPx !== SKELETON_UNMEASURED_WIDTH_PX) {
-			groupHeadingWidthsRef.current.set(groupId, widthPx);
-		}
-	}, []);
 	const memberSurfaceKind =
 		channel.type === ChannelTypes.GUILD_VOICE ? SkeletonMemberSurfaceKind.GUILD_VOICE : SkeletonMemberSurfaceKind.GUILD;
 	const rememberedMemberGroups = getRememberedSkeletonMemberGroups(channel.id, memberSurfaceKind);
@@ -255,7 +285,7 @@ const LazyMemberList = observer(function LazyMemberList({guild, channel}: LazyMe
 				memberSurfaceKind,
 				populatedGroups.map((group) => ({
 					rowCount: group.count,
-					headingWidthPx: groupHeadingWidthsRef.current.get(group.id) ?? SKELETON_UNMEASURED_WIDTH_PX,
+					headingWidthPx: groupHeadingWidths.get(group.id) ?? SKELETON_UNMEASURED_WIDTH_PX,
 				})),
 			);
 		},
@@ -511,7 +541,8 @@ const LazyMemberList = observer(function LazyMemberList({guild, channel}: LazyMe
 					virtualRows.push(
 						<div
 							key={`group-${rowIndex}-${group.id}`}
-							ref={(node) => registerGroupHeading(group.id, node)}
+							ref={registerGroupHeading}
+							data-member-group-id={group.id}
 							className={clsx(styles.virtualRow, styles.virtualGroupRow)}
 							style={rowStyle}
 							role="button"
@@ -533,7 +564,8 @@ const LazyMemberList = observer(function LazyMemberList({guild, channel}: LazyMe
 					virtualRows.push(
 						<div
 							key={`group-${rowIndex}-${group.id}`}
-							ref={(node) => registerGroupHeading(group.id, node)}
+							ref={registerGroupHeading}
+							data-member-group-id={group.id}
 							className={clsx(styles.virtualRow, styles.virtualGroupRow)}
 							style={rowStyle}
 							data-flx="channel.channel-members.lazy-member-list.virtual-row.group"
@@ -633,25 +665,21 @@ const GroupDMChannelMembers = observer(function GroupDMChannelMembers({channel}:
 	const allUserIds = currentUserId ? [currentUserId, ...channel.recipientIds] : channel.recipientIds;
 	const users = allUserIds.map((id) => Users.getUser(id)).filter((user): user is User => user != null);
 	const memberGroups = MemberListUtils.getGroupDMMemberGroups(users);
-	const memberGroupCountsKey = memberGroups.map((group) => `${group.id}:${group.count}`).join(',');
+	const memberGroupContentKey = memberGroups
-	const groupHeadingWidthsRef = useRef(new Map<string, number>());
+		.map((group) => `${group.id}:${group.displayName}:${group.count}`)
-	const registerGroupHeading = useCallback((groupId: string, node: HTMLDivElement | null) => {
+		.join(',');
-		const widthPx = measureMemberGroupHeadingWidthPx(node);
+	const {groupHeadingWidths, registerGroupHeading} = useGroupHeadingWidthTracking(memberGroupContentKey);
-		if (widthPx !== SKELETON_UNMEASURED_WIDTH_PX) {
-			groupHeadingWidthsRef.current.set(groupId, widthPx);
-		}
-	}, []);
 	useSkeletonLayoutReport(() => {
 		reportSkeletonMemberLayout(
 			channel.id,
 			SkeletonMemberSurfaceKind.GROUP_DM,
 			memberGroups.map((group) => ({
 				rowCount: group.count,
-				headingWidthPx: groupHeadingWidthsRef.current.get(group.id) ?? SKELETON_UNMEASURED_WIDTH_PX,
+				headingWidthPx: groupHeadingWidths.get(group.id) ?? SKELETON_UNMEASURED_WIDTH_PX,
 				subtextFlags: group.users.map((user) => hasVisibleCompactMemberCustomStatus(Presence.getCustomStatus(user.id))),
 			})),
 		);
-	}, `${channel.id}|${memberGroupCountsKey}`);
+	}, `${channel.id}|${memberGroupContentKey}`);
 	return (
 		<OutlineFrame hideTopBorder data-flx="channel.channel-members.outline-frame">
 			<MemberListContainer channelId={channel.id} data-flx="channel.channel-members.member-list-container">

fluxer_app/src/features/channel/components/channel_view/dm_channel_view/CallParticipantsRow.tsx

@@ -129,6 +129,7 @@ export const CallParticipantsRow = observer(
 			return () => resizeObserver.disconnect();
 		}, [participants.length]);
 		const avatarSize = layoutMetrics.avatarSize;
+		const scaledAvatarSize = avatarSize / getAppRemScale();
 		const callRippleStyle = useMemo(
 			() =>
 				({
@@ -175,7 +176,7 @@ export const CallParticipantsRow = observer(
 								>
 									<AvatarWithPresence
 										user={user}
-										size={avatarSize / getAppRemScale()}
+										size={scaledAvatarSize}
 										speaking={participantEntry?.speaking}
 										muted={participantEntry?.selfMute}
 										deafened={participantEntry?.selfDeaf}

fluxer_app/src/features/theme/hooks/useCustomThemeStyle.ts

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
 import {Logger} from '@app/features/platform/utils/AppLogger';
+import {clearRemScaleCache} from '@app/features/theme/layout/RemFromPx';
 import type {ThemeLibraryAsset, ThemeLibraryLocalFileReference} from '@app/features/theme/state/ThemeLibrary';
 import {resolveThemeCssReferences} from '@app/features/theme/utils/ThemeCssUtils';
 import {useEffect} from 'react';
@@ -32,10 +33,15 @@ export function useCustomThemeStyle({
 		if (!css) {
 			if (existing?.parentNode) {
 				existing.parentNode.removeChild(existing);
+				clearRemScaleCache();
 			}
 			return;
 		}
 		const styleElement = existing ?? document.createElement('style');
+		const applyThemeCss = (value: string): void => {
+			styleElement.textContent = value;
+			clearRemScaleCache();
+		};
 		styleElement.id = STYLE_ELEMENT_ID;
 		let disposed = false;
 		let objectUrls: Array<string> = [];
@@ -48,7 +54,7 @@ export function useCustomThemeStyle({
 					return;
 				}
 				objectUrls = resolved.objectUrls;
-				styleElement.textContent = resolved.css;
+				applyThemeCss(resolved.css);
 				if (resolved.missingAssetReferences.length > 0 || resolved.missingLocalFileReferences.length > 0) {
 					logger.warn('Custom theme CSS has unresolved file references', {
 						assets: resolved.missingAssetReferences,
@@ -58,7 +64,7 @@ export function useCustomThemeStyle({
 			})
 			.catch((error) => {
 				logger.error('Failed to resolve custom theme CSS references', error);
-				styleElement.textContent = css;
+				applyThemeCss(css);
 			});
 		if (!existing) {
 			document.head.appendChild(styleElement);

fluxer_app/src/features/theme/hooks/useRemScaleTracking.ts

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import {startRemScaleTracking} from '@app/features/theme/layout/RemFromPx';
+import {useLayoutEffect} from 'react';
+
+export function useRemScaleTracking(): void {
+	useLayoutEffect(() => startRemScaleTracking(document), []);
+}

fluxer_app/src/features/theme/layout/RemFromPx.test.ts

@@ -1,8 +1,101 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-import {REM_BASE_PX, remFromPx} from '@app/features/theme/layout/RemFromPx';
+import {
+	clearRemScaleCache,
+	getRemScaleForDocument,
+	REM_BASE_PX,
+	remFromPx,
+	startRemScaleTracking,
+} from '@app/features/theme/layout/RemFromPx';
 import {describe, expect, it} from 'vitest';
 
+interface StubDocumentHarness {
+	ownerDocument: Document;
+	setRootFontSize: (fontSize: string) => void;
+	getComputedStyleCallCount: () => number;
+	animationFrameRequestCount: () => number;
+	resizeListenerCount: () => number;
+	resolutionListenerCount: () => number;
+	watchedResolutionQueries: () => Array<string>;
+	dispatchResize: () => void;
+	setDevicePixelRatio: (devicePixelRatio: number) => void;
+	setInnerWidth: (innerWidth: number) => void;
+	setResolutionQueryMatches: (matches: boolean) => void;
+	dispatchResolutionChange: () => void;
+}
+
+function createStubDocumentHarness(initialRootFontSize: string): StubDocumentHarness {
+	let rootFontSize = initialRootFontSize;
+	let getComputedStyleCallCount = 0;
+	let animationFrameRequestCount = 0;
+	let resolutionQueryMatches = true;
+	const resizeListeners = new Set<() => void>();
+	const resolutionQueries = new Map<string, Set<() => void>>();
+	const documentElement = {} as HTMLElement;
+	const ownerWindow = {
+		devicePixelRatio: 1,
+		innerWidth: 1280,
+		getComputedStyle: () => {
+			getComputedStyleCallCount += 1;
+			return {fontSize: rootFontSize} as CSSStyleDeclaration;
+		},
+		requestAnimationFrame: () => {
+			animationFrameRequestCount += 1;
+			return 0;
+		},
+		cancelAnimationFrame: () => {},
+		addEventListener: (type: string, listener: () => void) => {
+			if (type !== 'resize') return;
+			resizeListeners.add(listener);
+		},
+		removeEventListener: (type: string, listener: () => void) => {
+			if (type !== 'resize') return;
+			resizeListeners.delete(listener);
+		},
+		matchMedia: (media: string) => {
+			const listeners = resolutionQueries.get(media) ?? new Set<() => void>();
+			resolutionQueries.set(media, listeners);
+			return {
+				media,
+				matches: resolutionQueryMatches,
+				addEventListener: (_type: string, listener: () => void) => listeners.add(listener),
+				removeEventListener: (_type: string, listener: () => void) => listeners.delete(listener),
+			} as unknown as MediaQueryList;
+		},
+	};
+	const ownerDocument = {documentElement, defaultView: ownerWindow} as unknown as Document;
+	return {
+		ownerDocument,
+		setRootFontSize: (fontSize) => {
+			rootFontSize = fontSize;
+		},
+		getComputedStyleCallCount: () => getComputedStyleCallCount,
+		animationFrameRequestCount: () => animationFrameRequestCount,
+		resizeListenerCount: () => resizeListeners.size,
+		resolutionListenerCount: () =>
+			[...resolutionQueries.values()].reduce((total, listeners) => total + listeners.size, 0),
+		watchedResolutionQueries: () =>
+			[...resolutionQueries.entries()].filter(([, listeners]) => listeners.size > 0).map(([media]) => media),
+		dispatchResize: () => {
+			for (const listener of [...resizeListeners]) listener();
+		},
+		setDevicePixelRatio: (devicePixelRatio) => {
+			ownerWindow.devicePixelRatio = devicePixelRatio;
+		},
+		setInnerWidth: (innerWidth) => {
+			ownerWindow.innerWidth = innerWidth;
+		},
+		setResolutionQueryMatches: (matches) => {
+			resolutionQueryMatches = matches;
+		},
+		dispatchResolutionChange: () => {
+			for (const listeners of [...resolutionQueries.values()]) {
+				for (const listener of [...listeners]) listener();
+			}
+		},
+	};
+}
+
 describe('remFromPx', () => {
 	it('uses a 16px base', () => {
 		expect(REM_BASE_PX).toBe(16);
@@ -32,3 +125,146 @@ describe('remFromPx', () => {
 		}
 	});
 });
+
+describe('getRemScaleForDocument', () => {
+	it('reports the root font size relative to the rem base', () => {
+		const defaultRoot = createStubDocumentHarness('16px');
+		const scaledRoot = createStubDocumentHarness('20px');
+		const zoomedRoot = createStubDocumentHarness('24px');
+		expect(getRemScaleForDocument(defaultRoot.ownerDocument)).toBe(1);
+		expect(getRemScaleForDocument(scaledRoot.ownerDocument)).toBe(1.25);
+		expect(getRemScaleForDocument(zoomedRoot.ownerDocument)).toBe(1.5);
+	});
+
+	it('reads the computed root font size once for repeated calls', () => {
+		const harness = createStubDocumentHarness('16px');
+		for (let call = 0; call < 200; call += 1) {
+			expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		}
+		expect(harness.getComputedStyleCallCount()).toBe(1);
+	});
+
+	it('never schedules per-frame invalidation work', () => {
+		const harness = createStubDocumentHarness('16px');
+		getRemScaleForDocument(harness.ownerDocument);
+		getRemScaleForDocument(harness.ownerDocument);
+		expect(harness.animationFrameRequestCount()).toBe(0);
+	});
+
+	it('re-reads after a resize that changes the device pixel ratio', () => {
+		const harness = createStubDocumentHarness('16px');
+		startRemScaleTracking(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setRootFontSize('20px');
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setDevicePixelRatio(1.25);
+		harness.dispatchResize();
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1.25);
+		expect(harness.watchedResolutionQueries()).toEqual(['(resolution: 1.25dppx)']);
+	});
+
+	it('re-reads after a resize that changes the viewport width', () => {
+		const harness = createStubDocumentHarness('16px');
+		startRemScaleTracking(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setRootFontSize('20px');
+		harness.setInnerWidth(720);
+		harness.dispatchResize();
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1.25);
+	});
+
+	it('ignores resizes that change neither the device pixel ratio nor the viewport width', () => {
+		const harness = createStubDocumentHarness('16px');
+		startRemScaleTracking(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setRootFontSize('20px');
+		for (let frame = 0; frame < 60; frame += 1) {
+			harness.dispatchResize();
+			expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		}
+		expect(harness.getComputedStyleCallCount()).toBe(1);
+	});
+
+	it('re-reads after a browser zoom changes the device pixel ratio', () => {
+		const harness = createStubDocumentHarness('16px');
+		startRemScaleTracking(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		expect(harness.watchedResolutionQueries()).toEqual(['(resolution: 1dppx)']);
+		harness.setRootFontSize('24px');
+		harness.setDevicePixelRatio(1.5);
+		harness.dispatchResolutionChange();
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1.5);
+		expect(harness.watchedResolutionQueries()).toEqual(['(resolution: 1.5dppx)']);
+		expect(harness.resolutionListenerCount()).toBe(1);
+	});
+
+	it('re-binds the resolution query from a resize when the query never matched', () => {
+		const harness = createStubDocumentHarness('16px');
+		harness.setResolutionQueryMatches(false);
+		startRemScaleTracking(harness.ownerDocument);
+		expect(harness.resolutionListenerCount()).toBe(0);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setRootFontSize('20px');
+		harness.dispatchResize();
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setDevicePixelRatio(1.5);
+		harness.setResolutionQueryMatches(true);
+		harness.dispatchResize();
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1.25);
+		expect(harness.watchedResolutionQueries()).toEqual(['(resolution: 1.5dppx)']);
+		expect(harness.resolutionListenerCount()).toBe(1);
+	});
+
+	it('re-reads after an explicit cache clear', () => {
+		const harness = createStubDocumentHarness('16px');
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		harness.setRootFontSize('12px');
+		clearRemScaleCache(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(0.75);
+		expect(harness.getComputedStyleCallCount()).toBe(2);
+	});
+
+	it('releases every listener when tracking is disposed', () => {
+		const harness = createStubDocumentHarness('16px');
+		const stopTracking = startRemScaleTracking(harness.ownerDocument);
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1);
+		expect(harness.resizeListenerCount()).toBe(1);
+		expect(harness.resolutionListenerCount()).toBe(1);
+		stopTracking();
+		expect(harness.resizeListenerCount()).toBe(0);
+		expect(harness.resolutionListenerCount()).toBe(0);
+		harness.setRootFontSize('20px');
+		expect(getRemScaleForDocument(harness.ownerDocument)).toBe(1.25);
+		expect(harness.resizeListenerCount()).toBe(0);
+		expect(harness.resolutionListenerCount()).toBe(0);
+	});
+
+	it('never registers listeners as a side effect of reading', () => {
+		const harness = createStubDocumentHarness('16px');
+		for (let call = 0; call < 5; call += 1) {
+			getRemScaleForDocument(harness.ownerDocument);
+		}
+		expect(harness.resizeListenerCount()).toBe(0);
+		expect(harness.resolutionListenerCount()).toBe(0);
+	});
+
+	it('invalidates every document when cleared without a target', () => {
+		const main = createStubDocumentHarness('16px');
+		const popout = createStubDocumentHarness('16px');
+		expect(getRemScaleForDocument(main.ownerDocument)).toBe(1);
+		expect(getRemScaleForDocument(popout.ownerDocument)).toBe(1);
+		main.setRootFontSize('20px');
+		popout.setRootFontSize('20px');
+		clearRemScaleCache();
+		expect(getRemScaleForDocument(main.ownerDocument)).toBe(1.25);
+		expect(getRemScaleForDocument(popout.ownerDocument)).toBe(1.25);
+	});
+
+	it('falls back to the rem base when the document has no view or no usable root font size', () => {
+		const detachedDocument = {documentElement: {} as HTMLElement, defaultView: null} as unknown as Document;
+		const brokenRoot = createStubDocumentHarness('not-a-length');
+		expect(getRemScaleForDocument(null)).toBe(1);
+		expect(getRemScaleForDocument(detachedDocument)).toBe(1);
+		expect(getRemScaleForDocument(brokenRoot.ownerDocument)).toBe(1);
+	});
+});

fluxer_app/src/features/theme/layout/RemFromPx.ts

@@ -17,10 +17,16 @@ export function remFromPx(px: number): `${number}rem` {
 	return `${rounded}rem`;
 }
 
-export function getRemScaleForDocument(ownerDocument: Document | null): number {
+interface CachedRemScale {
-	if (ownerDocument == null) {
+	scale: number;
-		return 1;
+	generation: number;
-	}
+}
+
+const cachedRemScales = new WeakMap<Document, CachedRemScale>();
+const remScaleDisposers = new WeakMap<Document, () => void>();
+let remScaleGeneration = 0;
+
+function readRemScaleFromDocument(ownerDocument: Document): number {
 	const ownerWindow = ownerDocument.defaultView;
 	if (ownerWindow == null) {
 		return 1;
@@ -31,3 +37,92 @@ export function getRemScaleForDocument(ownerDocument: Document | null): number {
 	}
 	return rootFontSize / REM_BASE_PX;
 }
+
+export function getRemScaleForDocument(ownerDocument: Document | null): number {
+	if (ownerDocument == null) {
+		return 1;
+	}
+	const cached = cachedRemScales.get(ownerDocument);
+	if (cached != null && cached.generation === remScaleGeneration) {
+		return cached.scale;
+	}
+	const scale = readRemScaleFromDocument(ownerDocument);
+	cachedRemScales.set(ownerDocument, {scale, generation: remScaleGeneration});
+	return scale;
+}
+
+export function clearRemScaleCache(ownerDocument?: Document | null): void {
+	if (ownerDocument == null) {
+		remScaleGeneration += 1;
+		return;
+	}
+	cachedRemScales.delete(ownerDocument);
+}
+
+export function startRemScaleTracking(ownerDocument: Document): () => void {
+	remScaleDisposers.get(ownerDocument)?.();
+	const ownerWindow = ownerDocument.defaultView;
+	if (ownerWindow == null) {
+		return () => {};
+	}
+	return trackRemScaleForWindow(ownerDocument, ownerWindow);
+}
+
+function trackRemScaleForWindow(ownerDocument: Document, ownerWindow: Window): () => void {
+	let resolutionQuery: MediaQueryList | null = null;
+	let trackedDevicePixelRatio = ownerWindow.devicePixelRatio;
+	let trackedInnerWidth = ownerWindow.innerWidth;
+
+	function invalidate(): void {
+		cachedRemScales.delete(ownerDocument);
+	}
+
+	function stopWatchingResolution(): void {
+		resolutionQuery?.removeEventListener('change', invalidateForRootFontSizeInputs);
+		resolutionQuery = null;
+	}
+
+	function watchResolution(): void {
+		if (typeof ownerWindow.matchMedia !== 'function') {
+			return;
+		}
+		const nextQuery = ownerWindow.matchMedia(`(resolution: ${ownerWindow.devicePixelRatio}dppx)`);
+		if (!nextQuery.matches) {
+			stopWatchingResolution();
+			return;
+		}
+		stopWatchingResolution();
+		resolutionQuery = nextQuery;
+		resolutionQuery.addEventListener('change', invalidateForRootFontSizeInputs);
+	}
+
+	function invalidateForRootFontSizeInputs(): void {
+		trackedDevicePixelRatio = ownerWindow.devicePixelRatio;
+		trackedInnerWidth = ownerWindow.innerWidth;
+		invalidate();
+		watchResolution();
+	}
+
+	function handleResize(): void {
+		if (ownerWindow.devicePixelRatio === trackedDevicePixelRatio && ownerWindow.innerWidth === trackedInnerWidth) {
+			return;
+		}
+		invalidateForRootFontSizeInputs();
+	}
+
+	ownerWindow.addEventListener('resize', handleResize);
+	watchResolution();
+	invalidate();
+
+	const dispose = (): void => {
+		if (remScaleDisposers.get(ownerDocument) !== dispose) {
+			return;
+		}
+		ownerWindow.removeEventListener('resize', handleResize);
+		stopWatchingResolution();
+		remScaleDisposers.delete(ownerDocument);
+		cachedRemScales.delete(ownerDocument);
+	};
+	remScaleDisposers.set(ownerDocument, dispose);
+	return dispose;
+}

fluxer_app/src/features/ui/utils/AppZoomUtils.ts

@@ -1,9 +1,6 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-import {getRemScaleForDocument} from '@app/features/theme/layout/RemFromPx';
+import {clearRemScaleCache, getRemScaleForDocument} from '@app/features/theme/layout/RemFromPx';
-
-const cachedRemScales = new WeakMap<Document, number>();
-const clearRemScaleCacheRafIds = new WeakMap<Document, number>();
 
 export interface AppZoomPoint {
 	x: number;
@@ -24,41 +21,12 @@ function getDefaultDocument(): Document | null {
 	return document;
 }
 
-function scheduleRemScaleCacheClear(ownerDocument: Document): void {
+export function clearAppZoomCache(): void {
-	const ownerWindow = ownerDocument.defaultView;
+	clearRemScaleCache();
-	if (ownerWindow == null || typeof ownerWindow.requestAnimationFrame !== 'function') {
-		cachedRemScales.delete(ownerDocument);
-		return;
-	}
-	if (clearRemScaleCacheRafIds.has(ownerDocument)) return;
-	const rafId = ownerWindow.requestAnimationFrame(() => {
-		clearRemScaleCacheRafIds.delete(ownerDocument);
-		cachedRemScales.delete(ownerDocument);
-	});
-	clearRemScaleCacheRafIds.set(ownerDocument, rafId);
-}
-
-export function clearAppZoomCache(ownerDocument: Document | null = getDefaultDocument()): void {
-	if (ownerDocument == null) return;
-	cachedRemScales.delete(ownerDocument);
-	const ownerWindow = ownerDocument.defaultView;
-	if (ownerWindow == null) return;
-	const remScaleRafId = clearRemScaleCacheRafIds.get(ownerDocument);
-	if (remScaleRafId == null) return;
-	if (typeof ownerWindow.cancelAnimationFrame === 'function') {
-		ownerWindow.cancelAnimationFrame(remScaleRafId);
-	}
-	clearRemScaleCacheRafIds.delete(ownerDocument);
 }
 
 export function getAppRemScale(ownerDocument: Document | null = getDefaultDocument()): number {
-	if (ownerDocument == null) return 1;
+	return getRemScaleForDocument(ownerDocument);
-	const cachedRemScale = cachedRemScales.get(ownerDocument);
-	if (cachedRemScale != null) return cachedRemScale;
-	const remScale = getRemScaleForDocument(ownerDocument);
-	cachedRemScales.set(ownerDocument, remScale);
-	scheduleRemScaleCacheClear(ownerDocument);
-	return remScale;
 }
 
 export function applyAppZoomToDocument(zoomPercent: number, electronApi?: AppZoomElectronApi | null): void {