Sync text filter keyword to URL query param

Extract a useQueryFilter hook that debounces the filter keyword and
mirrors it into the URL's q param via router.replace, so deep links
open pre-filtered and rapid typing doesn't flood history. Apply across
keys, files, find, and oslist, replacing their ad-hoc debounce logic.

src/app/os/files/page.tsx

@@ -2,7 +2,6 @@
 
 import { useState, useEffect, useMemo } from "react";
 import { useSearchParams } from "next/navigation";
-import { useDebounce } from "use-debounce";
 import { Search, X, ChevronsUpDown, ChevronsDownUp } from "lucide-react";
 
 import { Input } from "@/components/ui/input";
@@ -10,6 +9,7 @@ import { Button } from "@/components/ui/button";
 import FileSystem from "@/components/filesystem";
 import { HeaderPortal } from "@/components/header-portal";
 import { createEngine } from "@/lib/engine";
+import { useQueryFilter } from "@/hooks/use-query-filter";
 
 export default function Files() {
   const params = useSearchParams();
@@ -18,10 +18,9 @@ export default function Files() {
 
   const [loading, setLoading] = useState(true);
   const [files, setFiles] = useState<string[]>([]);
-  const [keyword, setKeyword] = useState("");
   const [expandAll, setExpandAll] = useState<boolean | null>(null);
 
-  const [debouncedKeyword] = useDebounce(keyword, 200);
+  const { keyword, setKeyword, debouncedKeyword } = useQueryFilter("q", 200);
 
   useEffect(() => {
     setLoading(true);

src/app/os/find/page.tsx

@@ -9,6 +9,7 @@ import { Button } from "@/components/ui/button";
 import FileSystem from "@/components/filesystem";
 import { HeaderPortal } from "@/components/header-portal";
 import { createEngine } from "@/lib/engine";
+import { useQueryFilter } from "@/hooks/use-query-filter";
 
 export default function FindByKey() {
   const params = useSearchParams();
@@ -29,17 +30,8 @@ export default function FindByKey() {
 
   const [loading, setLoading] = useState(true);
   const [paths, setPaths] = useState<string[]>([]);
-  const [keyword, setKeyword] = useState("");
-  const [debouncedKeyword, setDebouncedKeyword] = useState("");
   const [expandAll, setExpandAll] = useState<boolean | null>(null);
-
-  // Debounce keyword with 300ms delay
-  useEffect(() => {
-    const timer = setTimeout(() => {
-      setDebouncedKeyword(keyword);
-    }, 300);
-    return () => clearTimeout(timer);
-  }, [keyword]);
+  const { keyword, setKeyword, debouncedKeyword } = useQueryFilter();
 
   useEffect(() => {
     async function fetchPaths() {

src/app/os/keys/page.tsx

@@ -10,6 +10,7 @@ import { Button } from "@/components/ui/button";
 import { HeaderPortal } from "@/components/header-portal";
 import { createEngine } from "@/lib/engine";
 import { tokenizeKeys, getTopTokens } from "@/lib/tokenizer";
+import { useQueryFilter } from "@/hooks/use-query-filter";
 
 export default function Keys() {
   const params = useSearchParams();
@@ -18,15 +19,7 @@ export default function Keys() {
 
   const [loading, setLoading] = useState(true);
   const [keys, setKeys] = useState<string[]>([]);
-  const [keyword, setKeyword] = useState("");
-  const [debouncedKeyword, setDebouncedKeyword] = useState("");
-
-  useEffect(() => {
-    const timer = setTimeout(() => {
-      setDebouncedKeyword(keyword);
-    }, 300);
-    return () => clearTimeout(timer);
-  }, [keyword]);
+  const { keyword, setKeyword, debouncedKeyword } = useQueryFilter();
 
   useEffect(() => {
     async function load() {

src/components/oslist.tsx

@@ -5,6 +5,7 @@ import Link from "next/link";
 import { useSearchParams } from "next/navigation";
 import { useEffect, useMemo, useState } from "react";
 
+import { useQueryFilter } from "@/hooks/use-query-filter";
 import { dataURL } from "@/lib/env";
 import type { Group, OS } from "@/lib/types";
 import { HeaderPortal } from "./header-portal";
@@ -74,15 +75,7 @@ export default function OSList() {
   const [groups, setGroups] = useState<Group[]>([]);
   const [highlights, setHighlights] = useState<Set<string>>(new Set());
   const [selectedPlatform, setSelectedPlatform] = useState<string | null>(null);
-  const [keyword, setKeyword] = useState("");
-  const [debouncedKeyword, setDebouncedKeyword] = useState("");
-
-  useEffect(() => {
-    const timer = setTimeout(() => {
-      setDebouncedKeyword(keyword);
-    }, 200);
-    return () => clearTimeout(timer);
-  }, [keyword]);
+  const { keyword, setKeyword, debouncedKeyword } = useQueryFilter("q", 200);
 
   useEffect(() => {
     const set: Set<string> = new Set();

src/hooks/use-query-filter.ts

@@ -0,0 +1,45 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { useRouter, usePathname, useSearchParams } from "next/navigation";
+import { useDebounce } from "use-debounce";
+
+import { basePath } from "@/lib/env";
+
+/**
+ * Debounced text filter that mirrors its value into a URL query param.
+ *
+ * - Initializes the keyword from the param so deep links open pre-filtered.
+ * - Debounces, so rapid typing ("c" → "co" → "com") only writes the URL once.
+ * - Uses router.replace (not push), so intermediate filter states don't
+ *   pollute browser history.
+ */
+export function useQueryFilter(param = "q", delay = 300) {
+  const params = useSearchParams();
+  const router = useRouter();
+  const pathname = usePathname();
+
+  const [keyword, setKeyword] = useState(() => params.get(param) ?? "");
+  const [debouncedKeyword] = useDebounce(keyword, delay);
+
+  useEffect(() => {
+    const next = new URLSearchParams(params.toString());
+    if (debouncedKeyword) {
+      next.set(param, debouncedKeyword);
+    } else {
+      next.delete(param);
+    }
+
+    // usePathname() may include the configured basePath, which router.replace
+    // re-applies — strip it to avoid doubling (mirrors version-switcher).
+    const path =
+      basePath && pathname.startsWith(basePath)
+        ? pathname.slice(basePath.length)
+        : pathname;
+    const query = next.toString();
+    router.replace(query ? `${path}?${query}` : path, { scroll: false });
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [debouncedKeyword]);
+
+  return { keyword, setKeyword, debouncedKeyword };
+}