diff --git a/web/src/pages/Build.tsx b/web/src/pages/Build.tsx
index 5aada13..f5a0352 100644
--- a/web/src/pages/Build.tsx
+++ b/web/src/pages/Build.tsx
@@ -1,13 +1,23 @@
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { AudioLines, ChevronRight, Disc3, Folder, Link2, Play, Unlink } from "lucide-react";
+import { AudioLines, ChevronDown, ChevronRight, ChevronsDownUp, Disc3, Folder, FolderOpen, Link2, Play, Unlink } from "lucide-react";
 import { api, type Pair, type TreeEntry } from "../api";
 import { Alert, Badge, Button, Card, Empty, Modal, Spinner, cx, formatBytes } from "../ui";
 import { useToast } from "../toast";
 import { MiniPlayer, type Track } from "./Player";
+import { LetterBar } from "./Review";
+
+const rowID = (rel: string) => `build-${rel}`;
+
+// A row's checkbox is implied by a selected ancestor folder.
+const ancestorIn = (sel: Set<string>, rel: string) => {
+  for (let i = rel.lastIndexOf("/"); i > 0; i = rel.lastIndexOf("/", i - 1)) if (sel.has(rel.slice(0, i))) return true;
+  return false;
+};
 
 export function Build({ pair }: { pair: Pair }) {
-  const [path, setPath] = useState("");
+  // Open folders, oldest first, so Backspace closes the last one opened.
+  const [open, setOpen] = useState<string[]>([]);
   const [sel, setSel] = useState<Set<string>>(new Set());
   const [confirm, setConfirm] = useState<null | "build" | "remove">(null);
   // The mini-player keeps its own copy of the folder it started in.
@@ -16,8 +26,8 @@ export function Build({ pair }: { pair: Pair }) {
   const qc = useQueryClient();
   const toast = useToast();
   const tree = useQuery({
-    queryKey: ["tree", pair.id, path],
-    queryFn: () => api.get<TreeEntry[]>(`/pairs/${pair.id}/tree?path=${encodeURIComponent(path)}`),
+    queryKey: ["tree", pair.id, ""],
+    queryFn: () => api.get<TreeEntry[]>(`/pairs/${pair.id}/tree?path=`),
   });
   const run = useMutation({
     mutationFn: (kind: "build" | "remove") => api.post(`/pairs/${pair.id}/${kind}`, { paths: [...sel] }),
@@ -30,16 +40,42 @@ export function Build({ pair }: { pair: Pair }) {
   });
   const running = pair.job?.state === "running";
 
-  const crumbs = path ? path.split("/") : [];
   const toggle = (rel: string) =>
     setSel((s) => {
       const n = new Set(s);
-      n.has(rel) ? n.delete(rel) : n.add(rel);
+      if (n.has(rel)) n.delete(rel);
+      else {
+        // The folder covers everything inside it.
+        for (const x of n) if (x.startsWith(rel + "/")) n.delete(x);
+        n.add(rel);
+      }
       return n;
     });
+  const toggleOpen = (rel: string) =>
+    setOpen((o) => (o.includes(rel) ? o.filter((x) => x !== rel && !x.startsWith(rel + "/")) : [...o, rel]));
+  const play = (siblings: TreeEntry[], rel: string) => {
+    const queue = siblings.filter((x) => !x.dir).map((x) => ({ rel: x.rel, name: x.name }));
+    setPlayer({ queue, index: queue.findIndex((t) => t.rel === rel) });
+  };
   const all = tree.data ?? [];
   const allSelected = all.length > 0 && all.every((e) => sel.has(e.rel));
 
+  // Backspace closes the folder opened last, and scrolls back to it.
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key !== "Backspace" || !open.length || e.metaKey || e.ctrlKey || e.altKey) return;
+      const t = e.target as HTMLElement;
+      if (t.isContentEditable || t.tagName === "TEXTAREA" || (t.tagName === "INPUT" && (t as HTMLInputElement).type !== "checkbox")) return;
+      if (document.querySelector("[role=dialog]")) return;
+      e.preventDefault();
+      const last = open[open.length - 1];
+      setOpen(open.filter((x) => x !== last && !x.startsWith(last + "/")));
+      document.getElementById(rowID(last))?.scrollIntoView({ block: "nearest" });
+    };
+    window.addEventListener("keydown", onKey);
+    return () => window.removeEventListener("keydown", onKey);
+  }, [open]);
+
   return (
     <div className={cx("space-y-4", player && "pb-28")}>
       <Alert tone="blue">
@@ -49,19 +85,14 @@ export function Build({ pair }: { pair: Pair }) {
       </Alert>
 
       <div className="flex flex-wrap items-center justify-between gap-3">
-        <nav className="flex min-w-0 flex-wrap items-center gap-1 text-sm">
-          <button className="text-accent hover:underline cursor-pointer" onClick={() => setPath("")}>
-            master
-          </button>
-          {crumbs.map((c, i) => (
-            <span key={i} className="flex items-center gap-1">
-              <ChevronRight className="size-3.5 text-muted" />
-              <button className="truncate hover:underline cursor-pointer" onClick={() => setPath(crumbs.slice(0, i + 1).join("/"))}>
-                {c}
-              </button>
-            </span>
-          ))}
-        </nav>
+        <div className="flex items-center gap-3 text-xs text-muted">
+          <Button size="sm" variant="ghost" disabled={!open.length} onClick={() => setOpen([])}>
+            <ChevronsDownUp className="size-3.5" /> Collapse all
+          </Button>
+          <span className="hidden sm:inline">
+            <kbd className="rounded border border-line px-1 mono">Backspace</kbd> closes the last folder opened
+          </span>
+        </div>
         <div className="flex items-center gap-2">
           {sel.size > 0 && <span className="text-xs text-muted">{sel.size} selected</span>}
           <Button size="sm" disabled={!sel.size || running} onClick={() => setConfirm("remove")}>
@@ -73,7 +104,15 @@ export function Build({ pair }: { pair: Pair }) {
         </div>
       </div>
 
-      <Card className="overflow-hidden">
+      {all.length > 0 && (
+        <div className="sticky top-14 z-30 -mx-4 bg-bg/90 px-4 py-1.5 backdrop-blur">
+          <LetterBar
+            starts={all.map((e, i) => ({ letter: e.name.charAt(0), offset: i }))}
+            onJump={(i) => document.getElementById(rowID(all[i].rel))?.scrollIntoView({ block: "start", behavior: "smooth" })}
+          />
+        </div>
+      )}
+      <Card className="overflow-hidden [&>div:last-child]:border-b-0">
         {tree.isLoading && (
           <div className="p-8">
             <Spinner />
@@ -89,42 +128,14 @@ export function Build({ pair }: { pair: Pair }) {
               checked={allSelected}
               onChange={() => setSel(allSelected ? new Set() : new Set(all.map((e) => e.rel)))}
             />
-            Select all in this folder
+            Select everything in the master library
           </label>
         )}
-        {all.map((e) => (
-          <div key={e.rel} className={cx("flex items-center gap-3 border-b border-line px-4 py-2 last:border-0", sel.has(e.rel) && "bg-accent/5")}>
-            <input type="checkbox" className="" checked={sel.has(e.rel)} onChange={() => toggle(e.rel)} />
-            {e.dir ? (
-              <Folder className="size-4 shrink-0 text-accent" />
-            ) : (
-              <button
-                className={cx(
-                  "grid size-6 shrink-0 place-items-center rounded-full cursor-pointer",
-                  nowPlaying === e.rel ? "brand-gradient text-on-accent" : "text-muted hover:bg-accent/15 hover:text-accent",
-                )}
-                title={nowPlaying === e.rel ? "Playing" : "Listen"}
-                onClick={() => {
-                  const queue = all.filter((x) => !x.dir).map((x) => ({ rel: x.rel, name: x.name }));
-                  setPlayer({ queue, index: queue.findIndex((t) => t.rel === e.rel) });
-                }}
-              >
-                {nowPlaying === e.rel ? <AudioLines className="size-3.5" /> : <Play className="ml-px size-3 fill-current" />}
-              </button>
-            )}
-            {e.dir ? (
-              <button className="min-w-0 flex-1 truncate text-left text-sm hover:underline cursor-pointer" onClick={() => { setPath(e.rel); setSel(new Set()); }}>
-                {e.name}
-              </button>
-            ) : (
-              <span className={cx("min-w-0 flex-1 truncate text-sm", nowPlaying === e.rel && "font-medium text-accent")}>{e.name}</span>
-            )}
-            {e.dir && !!e.linked && <Badge tone="green">{e.linked} linked</Badge>}
-            {e.state === "linked" && <Badge tone="green">in slave</Badge>}
-            {e.state === "conflict" && <Badge tone="amber">different file in slave</Badge>}
-            {!e.dir && <span className="w-20 text-right text-xs tabular-nums text-muted">{formatBytes(e.size ?? 0)}</span>}
-          </div>
-        ))}
+        <TreeRows
+          entries={all}
+          depth={0}
+          ctx={{ pairId: pair.id, open, sel, nowPlaying, toggle, toggleOpen, play }}
+        />
       </Card>
       <p className="text-xs text-muted">Folder link counts come from the last scan. Song states are live.</p>
 
@@ -164,3 +175,91 @@ export function Build({ pair }: { pair: Pair }) {
     </div>
   );
 }
+
+type TreeCtx = {
+  pairId: number;
+  open: string[];
+  sel: Set<string>;
+  nowPlaying: string | null;
+  toggle: (rel: string) => void;
+  toggleOpen: (rel: string) => void;
+  play: (siblings: TreeEntry[], rel: string) => void;
+};
+
+function TreeRows({ entries, depth, ctx }: { entries: TreeEntry[]; depth: number; ctx: TreeCtx }) {
+  return entries.map((e) => <TreeRow key={e.rel} e={e} siblings={entries} depth={depth} ctx={ctx} />);
+}
+
+function TreeRow({ e, siblings, depth, ctx }: { e: TreeEntry; siblings: TreeEntry[]; depth: number; ctx: TreeCtx }) {
+  const { open, sel, nowPlaying } = ctx;
+  const isOpen = e.dir && open.includes(e.rel);
+  const inherited = ancestorIn(sel, e.rel);
+  const checked = inherited || sel.has(e.rel);
+  const children = useQuery({
+    queryKey: ["tree", ctx.pairId, e.rel],
+    queryFn: () => api.get<TreeEntry[]>(`/pairs/${ctx.pairId}/tree?path=${encodeURIComponent(e.rel)}`),
+    enabled: isOpen,
+  });
+  return (
+    <>
+      <div
+        id={rowID(e.rel)}
+        className={cx("flex scroll-mt-28 items-center gap-3 border-b border-line py-2 pr-4", checked && "bg-accent/5")}
+        style={{ paddingLeft: 16 + depth * 24 }}
+      >
+        <input
+          type="checkbox"
+          className=""
+          checked={checked}
+          disabled={inherited}
+          title={inherited ? "Included with a selected folder" : undefined}
+          onChange={() => ctx.toggle(e.rel)}
+        />
+        {e.dir ? (
+          <button className="flex shrink-0 items-center gap-1 text-accent cursor-pointer" onClick={() => ctx.toggleOpen(e.rel)} aria-expanded={isOpen} aria-label={isOpen ? "Collapse" : "Expand"}>
+            {isOpen ? <ChevronDown className="size-3.5 text-muted" /> : <ChevronRight className="size-3.5 text-muted" />}
+            {isOpen ? <FolderOpen className="size-4" /> : <Folder className="size-4" />}
+          </button>
+        ) : (
+          <button
+            className={cx(
+              "ml-[18px] grid size-6 shrink-0 place-items-center rounded-full cursor-pointer",
+              nowPlaying === e.rel ? "brand-gradient text-on-accent" : "text-muted hover:bg-accent/15 hover:text-accent",
+            )}
+            title={nowPlaying === e.rel ? "Playing" : "Listen"}
+            onClick={() => ctx.play(siblings, e.rel)}
+          >
+            {nowPlaying === e.rel ? <AudioLines className="size-3.5" /> : <Play className="ml-px size-3 fill-current" />}
+          </button>
+        )}
+        {e.dir ? (
+          <button className="min-w-0 flex-1 truncate text-left text-sm hover:underline cursor-pointer" onClick={() => ctx.toggleOpen(e.rel)}>
+            {e.name}
+          </button>
+        ) : (
+          <span className={cx("min-w-0 flex-1 truncate text-sm", nowPlaying === e.rel && "font-medium text-accent")}>{e.name}</span>
+        )}
+        {e.dir && !!e.linked && <Badge tone="green">{e.linked} linked</Badge>}
+        {e.state === "linked" && <Badge tone="green">in slave</Badge>}
+        {e.state === "conflict" && <Badge tone="amber">different file in slave</Badge>}
+        {!e.dir && <span className="w-20 text-right text-xs tabular-nums text-muted">{formatBytes(e.size ?? 0)}</span>}
+      </div>
+      {isOpen && children.isLoading && (
+        <div className="border-b border-line py-2 text-xs text-muted" style={{ paddingLeft: 16 + (depth + 1) * 24 }}>
+          Loading…
+        </div>
+      )}
+      {isOpen && children.error && (
+        <div className="border-b border-line py-2 pr-4" style={{ paddingLeft: 16 + (depth + 1) * 24 }}>
+          <Alert tone="red">{children.error.message}</Alert>
+        </div>
+      )}
+      {isOpen && children.data?.length === 0 && (
+        <div className="border-b border-line py-2 text-xs text-muted" style={{ paddingLeft: 16 + (depth + 1) * 24 }}>
+          Empty folder
+        </div>
+      )}
+      {isOpen && children.data && <TreeRows entries={children.data} depth={depth + 1} ctx={ctx} />}
+    </>
+  );
+}