fix: stop icons remounting and shelves collapsing into nothing

frontend/src/lib/components/layout/nav-main.svelte

@@ -3,58 +3,74 @@
     import * as Collapsible from '$lib/components/ui/collapsible/index.js';
     import * as Sidebar from '$lib/components/ui/sidebar/index.js';
     import { Badge } from "$lib/components/ui/badge/index.js";
+    import { useSidebar } from '$lib/components/ui/sidebar/index.js';
     import { getBookshelfState } from '$lib/state/bookshelf.svelte';
     import { getLibraryState } from '$lib/state/library.svelte';
     import { House, LibraryBig, Rows3, ChevronRightIcon } from '@lucide/svelte';
 
     const libraryState = getLibraryState();
     const bookshelfState = getBookshelfState();
+    const sidebar = useSidebar();
 
-    let items = $derived(
-        [
-            {
-                title: 'Home',
-                url: `/library/${libraryState.activeLibrary?.id}`,
-                icon: House
-            },
-            {
-                title: 'Library',
-                url: `/library/${libraryState.activeLibrary?.id}/view`,
-                icon: LibraryBig
-            },
-            {
-                title: 'Shelves',
-                url: '#',
-                icon: Rows3,
-                shelves: []
-            }
-        ].map((item) => ({
-            ...item,
-            isActive: page.url.pathname === item.url
-        }))
-    );
+    // Owned here so the collapsed rail can force it open when it expands the
+    // sidebar. Previously open={isActive}, which was always false — the Shelves
+    // item has no route of its own, so pathname never matched.
+    let shelvesOpen = $state(false);
+
+    // Deliberately a plain const, not $derived. These objects hold component
+    // references, and `<item.icon />` is a dynamic component — if the array were
+    // rebuilt on every navigation the icons would remount, flashing and shifting
+    // layout. Only the url and active state need to be reactive, so they are
+    // computed per-item in the markup instead.
+    const items = [
+        { title: 'Home', icon: House, path: (id?: number) => `/library/${id}` },
+        { title: 'Library', icon: LibraryBig, path: (id?: number) => `/library/${id}/view` },
+        { title: 'Shelves', icon: Rows3, path: () => '#', shelves: [] }
+    ];
 </script>
 
-<Sidebar.Group>
-    <Sidebar.Menu>
-        {#each items as item (item.title)}
-            {#if 'shelves' in item}
-                <Collapsible.Root open={item.isActive} class="group/collapsible">
-                    {#snippet child({ props })}
-                        <Sidebar.MenuItem {...props}>
-                            <Collapsible.Trigger>
-                                {#snippet child({ props })}
-                                    <Sidebar.MenuButton {...props} tooltipContent={item.title} class="h-10">
-                                        {#if item.icon}
+{#snippet shelvesTrigger(item: { title: string; icon: typeof House })}
     <item.icon class="scale-125" />
-                                        {/if}
     <span class="text-md ml-2">{item.title}</span>
     <ChevronRightIcon
         class="ml-auto transition-transform duration-200 group-data-[state=open]/collapsible:rotate-90"
     />
+{/snippet}
+
+<Sidebar.Group>
+    <Sidebar.Menu>
+        {#each items as item (item.title)}
+            {@const url = item.path(libraryState.activeLibrary?.id)}
+            {@const isActive = page.url.pathname === url}
+            {#if 'shelves' in item}
+                <Collapsible.Root bind:open={shelvesOpen} class="group/collapsible">
+                    {#snippet child({ props })}
+                        <Sidebar.MenuItem {...props}>
+                            {#if sidebar.state === 'collapsed'}
+                                <!--
+                                    Sidebar.MenuSub is group-data-[collapsible=icon]:hidden, so
+                                    toggling in the rail opens a list nobody can see. Expand the
+                                    sidebar and open the section instead of toggling.
+                                -->
+                                <Sidebar.MenuButton
+                                    tooltipContent={item.title}
+                                    class="h-10"
+                                    onclick={() => {
+                                        sidebar.setOpen(true);
+                                        shelvesOpen = true;
+                                    }}
+                                >
+                                    {@render shelvesTrigger(item)}
+                                </Sidebar.MenuButton>
+                            {:else}
+                                <Collapsible.Trigger>
+                                    {#snippet child({ props })}
+                                        <Sidebar.MenuButton {...props} tooltipContent={item.title} class="h-10">
+                                            {@render shelvesTrigger(item)}
                                         </Sidebar.MenuButton>
                                     {/snippet}
                                 </Collapsible.Trigger>
+                            {/if}
                             <Collapsible.Content>
                                 <Sidebar.MenuSub class="w-full">
                                         {#each bookshelfState.getBookshelves(libraryState.activeLibrary!.id) ?? [] as shelf (shelf.id)}
@@ -85,9 +101,9 @@
                 </Collapsible.Root>
             {:else}
                 <Sidebar.MenuItem>
-                    <Sidebar.MenuButton isActive={item.isActive} tooltipContent={item.title} class="h-10">
+                    <Sidebar.MenuButton isActive={isActive} tooltipContent={item.title} class="h-10">
                         {#snippet child({ props })}
-                            <a href={item.url} {...props}>
+                            <a href={url} {...props}>
                                 {#if item.icon}
                                     <item.icon class="scale-125" />
                                 {/if}

frontend/src/lib/components/layout/theme-toggle.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { toggleMode, mode } from 'mode-watcher';
+	import { toggleMode } from 'mode-watcher';
 
 	import * as Tooltip from '$lib/components/ui/tooltip/index.js';
 	import { buttonVariants } from '$lib/components/ui/button/button.svelte';
@@ -9,25 +9,26 @@
 	let { class: className = '' }: { class?: string } = $props();
 </script>
 
+<!--
+	Which icon shows is decided by CSS off the `dark` class, not by reading
+	`mode.current` in the markup. The server cannot know the mode, so a
+	`{#if mode.current === 'light'}` renders the wrong branch during SSR and
+	visibly swaps on hydration.
+-->
 <Tooltip.Provider>
 	<Tooltip.Root>
 		<Tooltip.Trigger
 			onclick={toggleMode}
 			class="{buttonVariants({ variant: 'ghost', size: 'icon' })} scale-110 {className}"
 		>
-			{#if mode.current === 'light'}
-				<Moon class="scale-110" />
-			{:else}
-				<Sun class="scale-110" />
-			{/if}
+			<Moon class="scale-110 dark:hidden" />
+			<Sun class="hidden scale-110 dark:block" />
+			<span class="sr-only">Toggle light and dark mode</span>
 		</Tooltip.Trigger>
 
 		<Tooltip.Content>
-			{#if mode.current === 'light'}
-				<p>Dark Mode</p>
-			{:else}
-				<p>Light Mode</p>
-			{/if}
+			<p class="dark:hidden">Dark Mode</p>
+			<p class="hidden dark:block">Light Mode</p>
 		</Tooltip.Content>
 	</Tooltip.Root>
 </Tooltip.Provider>

frontend/src/lib/components/view/book-list.svelte

@@ -23,8 +23,10 @@
 		// Start observing
 		resizeObserver.observe(scrollContainer);
 
-		// Initial check with delay
-		setTimeout(updateScrollState, 0);
+		// Measure synchronously. Reading scrollWidth/clientWidth forces layout, so
+		// the numbers are already accurate here — deferring to a macrotask just
+		// guaranteed one painted frame with the arrows in the wrong state.
+		updateScrollState();
 
 		// Cleanup
 		return () => {
@@ -33,12 +35,14 @@
 	});
 
 	const scrollLeft = () => {
+		if (!scrollContainer) return;
 		scrollContainer.scrollBy({ left: -scrollContainer.clientWidth, behavior: 'smooth' });
 		// Update state after scroll
 		setTimeout(updateScrollState, 100);
 	};
 
 	const scrollRight = () => {
+		if (!scrollContainer) return;
 		scrollContainer.scrollBy({ left: scrollContainer.clientWidth, behavior: 'smooth' });
 		// Update state after scroll
 		setTimeout(updateScrollState, 100);
@@ -61,9 +65,13 @@
 {#if books.length > 0}
 	<div class="flex w-full flex-col">
 		<div class="flex items-center">
-			<h1 class="ml-4 text-xl font-semibold">{title}</h1>
-			{#if needsScroll}
-				<div class="ml-auto">
+			<h1 class="ml-4 font-serif text-xl font-semibold">{title}</h1>
+			<!--
+				Always mounted, only hidden. `needsScroll` is measured after mount,
+				so mounting on it made the arrows pop in a frame late and shift the
+				header; `invisible` keeps the space reserved from the first paint.
+			-->
+			<div class="ml-auto {needsScroll ? '' : 'invisible'}" aria-hidden={!needsScroll}>
 				<button
 					type="button"
 					disabled={!canScrollLeft}
@@ -83,7 +91,6 @@
 					<ChevronRight size="20" strokeWidth="3" />
 				</button>
 			</div>
-			{/if}
 		</div>
 		<div
 			bind:this={scrollContainer}