diff --git a/apps/app-frontend/src/components/ui/QuickInstanceSwitcher.vue b/apps/app-frontend/src/components/ui/QuickInstanceSwitcher.vue
index 256a0d0121..6d41adb369 100644
--- a/apps/app-frontend/src/components/ui/QuickInstanceSwitcher.vue
+++ b/apps/app-frontend/src/components/ui/QuickInstanceSwitcher.vue
@@ -5,7 +5,9 @@ import {
 	commonMessages,
 	ContextMenu,
 	defineMessages,
+	injectAuth,
 	injectNotificationManager,
+	injectUserPreferences,
 	useVIntl,
 } from '@modrinth/ui'
 import { useQuery } from '@tanstack/vue-query'
@@ -15,6 +17,7 @@ import { useRouter } from 'vue-router'
 
 import NavButton from '@/components/ui/NavButton.vue'
 import { useAppEvent } from '@/composables/use-app-event'
+import { useAppSettings } from '@/composables/use-app-settings.ts'
 import { handleSevereError } from '@/composables/use-error.js'
 import {
 	QUICK_INSTANCE_LIMIT_MAX,
@@ -28,7 +31,11 @@ import { showInstanceInFolder } from '@/helpers/utils'
 import { instanceListQueryOptions } from '@/pages/instance/query-options'
 
 const ITEM_SIZE = 52
+const SYNC_DEBOUNCE_MS = 1000
 const { handleError } = injectNotificationManager()
+const auth = injectAuth()
+const { updatePreferences } = injectUserPreferences()
+const appSettings = useAppSettings()
 const instancesQuery = useQuery(instanceListQueryOptions())
 const router = useRouter()
 const instanceOptions = ref()
@@ -97,6 +104,41 @@ const setLimit = (count) => {
 	}
 }
 
+const syncLimitToAccount = () => {
+	if (!appSettings.syncFeaturesAcrossDevices || !auth.user.value) {
+		return
+	}
+	updatePreferences({
+		behavior: {
+			quick_instance_count: quickInstances.limit.value ?? QUICK_INSTANCE_LIMIT_MAX,
+		},
+	}).catch(() => undefined)
+}
+
+let syncTimeout = null
+
+const cancelScheduledSync = () => {
+	if (syncTimeout !== null) {
+		clearTimeout(syncTimeout)
+		syncTimeout = null
+	}
+}
+
+const scheduleSync = () => {
+	cancelScheduledSync()
+	syncTimeout = setTimeout(() => {
+		syncTimeout = null
+		syncLimitToAccount()
+	}, SYNC_DEBOUNCE_MS)
+}
+
+const flushScheduledSync = () => {
+	if (syncTimeout !== null) {
+		cancelScheduledSync()
+		syncLimitToAccount()
+	}
+}
+
 const nudgeLimit = (delta) => {
 	if (!canDrag.value) {
 		return
@@ -105,11 +147,17 @@ const nudgeLimit = (delta) => {
 	if (target < 0 || target > maxVisible.value) {
 		flashOverdrag()
 	}
+	const previousLimit = quickInstances.limit.value
 	setLimit(target)
+	if (quickInstances.limit.value !== previousLimit) {
+		scheduleSync()
+	}
 }
 
 let dragStartY = 0
 let dragStartCount = 0
+let dragStartLimit = null
+let dragHadPendingSync = false
 let wasOverdragging = false
 let overdragTimeout = null
 
@@ -142,6 +190,9 @@ const onDividerPointerDown = (event) => {
 	clearOverdragFlash()
 	dragStartY = event.clientY
 	dragStartCount = visibleCount.value
+	dragStartLimit = quickInstances.limit.value
+	dragHadPendingSync = syncTimeout !== null
+	cancelScheduledSync()
 	document.body.classList.add('quick-instance-dragging')
 	event.currentTarget.setPointerCapture(event.pointerId)
 }
@@ -171,6 +222,9 @@ const endDrag = (event) => {
 	if (event?.currentTarget?.hasPointerCapture?.(event.pointerId)) {
 		event.currentTarget.releasePointerCapture(event.pointerId)
 	}
+	if (dragHadPendingSync || quickInstances.limit.value !== dragStartLimit) {
+		syncLimitToAccount()
+	}
 }
 
 const onDividerPointerUp = (event) => {
@@ -191,6 +245,7 @@ onMounted(() => {
 
 onUnmounted(() => {
 	resizeObserver?.disconnect()
+	flushScheduledSync()
 	document.body.classList.remove('quick-instance-dragging')
 	clearOverdragFlash()
 })
diff --git a/packages/app-lib/src/state/cache.rs b/packages/app-lib/src/state/cache.rs
index da0c73a39f..43958ec282 100644
--- a/packages/app-lib/src/state/cache.rs
+++ b/packages/app-lib/src/state/cache.rs
@@ -443,18 +443,25 @@ pub struct KnownModrinthFile<'a> {
     pub version_id: &'a str,
 }
 
+/// Returns the file's modification time as signed nanoseconds since the Unix
+/// epoch, reinterpreted as `u64` so pre-1970 timestamps stay distinct from one
+/// another and cast back to the correct negative `i64`.
 pub(crate) fn file_modified_at_ns(
     metadata: &std::fs::Metadata,
 ) -> std::io::Result<u64> {
-    let elapsed = metadata
+    let signed_ns = match metadata
         .modified()?
         .duration_since(std::time::UNIX_EPOCH)
-        .map_err(std::io::Error::other)?;
+    {
+        Ok(since_epoch) => {
+            i64::try_from(since_epoch.as_nanos()).unwrap_or(i64::MAX)
+        }
+        Err(before_epoch) => i64::try_from(before_epoch.duration().as_nanos())
+            .map(|ns| -ns)
+            .unwrap_or(i64::MIN),
+    };
 
-    Ok(elapsed
-        .as_secs()
-        .saturating_mul(1_000_000_000)
-        .saturating_add(u64::from(elapsed.subsec_nanos())))
+    Ok(signed_ns as u64)
 }
 
 pub(crate) fn file_hash_cache_key(