Virtualize the file tree's root entries

The file tree was the one list left un-virtualized, so wide directories
mounted every entry and re-rendered all rows on selection. Render
entries through VirtualList like the other lists.

src/components/FileTreeFolder.svelte

@@ -11,6 +11,10 @@
     currentPath: string;
     name: string;
     prefix: string;
+    // Expansion state lives in Tree.svelte so it survives the virtualizer
+    // unmounting off-screen rows.
+    isExpanded: (prefix: string) => boolean;
+    toggleExpanded: (prefix: string) => void;
     indent?: number;
   }
 
@@ -19,10 +23,12 @@
     fetchBlob,
     currentPath,
     prefix,
+    isExpanded,
+    toggleExpanded,
     fetchTree,
     indent = 0.5,
   }: Props = $props();
-  let expanded = $derived(currentPath.indexOf(prefix) === 0);
+  const expanded = $derived(isExpanded(prefix));
 
   const treePromise = $derived(
     expanded ? fetchTree(prefix) : Promise.resolve(undefined),
@@ -46,7 +52,7 @@
   class="folder"
   style:padding-left="{indent}rem"
   style:padding-right="0.5rem"
-  onclick={() => (expanded = !expanded)}>
+  onclick={() => toggleExpanded(prefix)}>
   <div class="global-flex txt-body-m-regular" style:padding="0.25rem 0">
     <div class:txt-missing={!expanded}>
       <Icon name={expanded ? "folder-open" : "folder"} />
@@ -68,6 +74,8 @@
               {fetchBlob}
               name={entry.name}
               {currentPath}
+              {isExpanded}
+              {toggleExpanded}
               prefix={`${entry.path}/`}
               indent={indent + 1.5} />
           {:else if entry.kind === "blob"}

src/components/Tree.svelte

@@ -1,8 +1,11 @@
 <script lang="ts">
   import type { Tree } from "@bindings/source/Tree";
 
+  import { SvelteMap } from "svelte/reactivity";
+
   import FileTreeFile from "@app/components/FileTreeFile.svelte";
   import FileTreeFolder from "@app/components/FileTreeFolder.svelte";
+  import VirtualList from "@app/components/VirtualList.svelte";
 
   interface Props {
     tree: Tree;
@@ -12,15 +15,40 @@
   }
 
   const { currentPath, tree, fetchTree, fetchBlob }: Props = $props();
+
+  // Manual expand/collapse overrides, keyed by folder prefix. Held here
+  // rather than in FileTreeFolder so the state survives the virtualizer
+  // unmounting off-screen rows; without an override a folder is expanded
+  // when it contains the open file.
+  const expandedOverrides = new SvelteMap<string, boolean>();
+  $effect(() => {
+    // Navigating to another file resets every folder to its default, the
+    // same behaviour the per-folder writable $derived had before the state
+    // was hoisted.
+    // eslint-disable-next-line @typescript-eslint/no-unused-expressions
+    currentPath;
+    expandedOverrides.clear();
+  });
+  function isExpanded(prefix: string): boolean {
+    return expandedOverrides.get(prefix) ?? currentPath.indexOf(prefix) === 0;
+  }
+  function toggleExpanded(prefix: string) {
+    expandedOverrides.set(prefix, !isExpanded(prefix));
+  }
 </script>
 
-<div style:display="flex" style:flex-direction="column" style:gap="0.25rem">
-  {#each tree.entries as entry (entry.name)}
+<VirtualList
+  items={tree.entries}
+  getKey={entry => entry.path}
+  estimatedItemSize={28}>
+  {#snippet row(entry)}
     {#if entry.kind === "tree"}
       <FileTreeFolder
         name={entry.name}
         prefix={`${entry.path}/`}
         {currentPath}
+        {isExpanded}
+        {toggleExpanded}
         {fetchTree}
         {fetchBlob} />
     {:else}
@@ -29,5 +57,5 @@
         fetchBlob={() => fetchBlob(entry.path)}
         active={entry.path === currentPath} />
     {/if}
-  {/each}
-</div>
+  {/snippet}
+</VirtualList>