feat: narrow a library page as the reader types in its search box

app/lib/application/controllers/browser_controller.dart

@@ -254,6 +254,27 @@ class BrowserController {
     await _loadCurrent();
   }
 
+  /// Narrows the open page to what the reader has typed in a search box. The
+  /// query rides in the address as it does when the box is submitted, but it
+  /// replaces the step of the walk they are standing on instead of adding
+  /// one, so a search of eight letters is one step back and not eight.
+  ///
+  /// Only one of our own pages narrows this way. A form on a fetched page is
+  /// sent when it is submitted, never while it is being filled in, and so is
+  /// a form that changes something rather than going somewhere.
+  Future<void> narrowWhileTyping(String action, Map<String, String> fields) async {
+    if (!MewsUrls.isInternal(action) || MewsUrls.isAction(action)) return;
+    if (_stackIndex < 0) return;
+    final url = MewsUrls.page(Uri.parse(action), fields);
+    if (url == _stack[_stackIndex]) return;
+    // The page the reader was going to next belonged to the search they have
+    // now changed, so it is dropped as it is on any other navigation.
+    _stack.removeRange(_stackIndex + 1, _stack.length);
+    _stack[_stackIndex] = url;
+    _updateNeighbors();
+    await _loadCurrent();
+  }
+
   Future<void> submitForm(String action, String method, Map<String, String> fields) async {
     // A form on one of our own pages is a navigation or an action, never a
     // network request; the fetch client cannot speak the mews scheme.

app/lib/presentation/reader/mews_page_view.dart

@@ -1,3 +1,5 @@
+import 'dart:async' show Timer;
+
 import 'dart:typed_data';
 
 import 'package:flutter/services.dart'
@@ -15,6 +17,10 @@ import '../../application/fonts.dart';
 import '../../mews/document.dart';
 import 'palettes.dart';
 
+/// How long the reader has to stop typing before a search box narrows its
+/// page. Long enough that a word is one query rather than one per letter.
+const searchTypingPause = Duration(milliseconds: 250);
+
 /// Renders a parsed [MewsPage] with the client's own presentation, following
 /// the measurements of the default stylesheet mews-0.1.css: a 40em text
 /// column, 1.5 line height, and the block margins authors expect.
@@ -27,6 +33,7 @@ class MewsPageView extends StatefulWidget {
     required this.visitedUrls,
     required this.onLinkTap,
     required this.onFormSubmit,
+    this.onFormChange,
     this.fetchImageBytes,
     this.scrollController,
     this.ownPage = false,
@@ -41,6 +48,10 @@ class MewsPageView extends StatefulWidget {
   final void Function(String url) onLinkTap;
   final void Function(FormBlock form, Map<String, String> values) onFormSubmit;
 
+  /// Called as the reader types, so a search box can narrow its page without
+  /// being submitted. Null on a page whose forms only answer when submitted.
+  final void Function(FormBlock form, Map<String, String> values)? onFormChange;
+
   /// The bytes of an image living on a protocol page, where no address exists
   /// that the platform could load from. Null when the page holds none.
   final Future<Uint8List?> Function(String url)? fetchImageBytes;
@@ -412,10 +423,15 @@ class _MewsPageViewState extends State<MewsPageView> {
       ),
       FormBlock() => Padding(
         padding: EdgeInsets.only(bottom: base),
+        // Keyed by what it submits to: a page rebuilt as the reader types in
+        // this box is matched up with the box they are typing in, so the text
+        // and the cursor stay where they are.
+        key: ValueKey('form:${block.action}'),
         child: _MewsForm(
           form: block,
           foreground: foreground,
           onFormSubmit: widget.onFormSubmit,
+          onFormChange: widget.onFormChange,
           ownPage: widget.ownPage,
         ),
       ),
@@ -669,12 +685,14 @@ class _MewsForm extends StatefulWidget {
     required this.form,
     required this.foreground,
     required this.onFormSubmit,
+    this.onFormChange,
     this.ownPage = false,
   });
 
   final FormBlock form;
   final TextStyle foreground;
   final void Function(FormBlock form, Map<String, String> values) onFormSubmit;
+  final void Function(FormBlock form, Map<String, String> values)? onFormChange;
 
   /// On our own pages a form is a search box or a one-field prompt, so it sits
   /// on one line instead of filling the column like a page's own form.
@@ -688,8 +706,37 @@ class _MewsFormState extends State<_MewsForm> {
   final _controllers = <String, TextEditingController>{};
   final _values = <String, String>{};
 
+  /// Waits out [searchTypingPause] before the page is narrowed.
+  Timer? _typing;
+
+  /// A page that narrows as the reader types is rebuilt under the box they are
+  /// typing in, and the rebuilt box carries what they typed, so the text is
+  /// left alone. When the page changes it itself, as clearing a search does,
+  /// the box follows.
+  @override
+  void didUpdateWidget(_MewsForm oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    final wasValues = _modelValues(oldWidget.form);
+    _modelValues(widget.form).forEach((name, value) {
+      if (wasValues[name] == value) return;
+      final controller = _controllers[name];
+      if (controller == null || controller.text == value) return;
+      controller.value = TextEditingValue(
+        text: value,
+        selection: TextSelection.collapsed(offset: value.length),
+      );
+      _values[name] = value;
+    });
+  }
+
+  Map<String, String> _modelValues(FormBlock form) => {
+        for (final field in form.fields)
+          if (field case MewsTextField(:final name, :final value)) name: value,
+      };
+
   @override
   void dispose() {
+    _typing?.cancel();
     for (final controller in _controllers.values) {
       controller.dispose();
     }
@@ -768,6 +815,7 @@ class _MewsFormState extends State<_MewsForm> {
           maxLength: maxLength,
           style: style,
           decoration: InputDecoration(labelText: field.label, isDense: true),
+          onChanged: (text) => _changed(widget.form, name, text),
         ),
       MewsTextAreaField(:final name, :final value, :final rows) =>
         TextFormField(
@@ -832,6 +880,23 @@ class _MewsFormState extends State<_MewsForm> {
   }
 
   void _submit(FormBlock form) {
+    // Submitting says the reader is done typing, so it stands in for the
+    // narrowing they were waiting on.
+    _typing?.cancel();
+    widget.onFormSubmit(form, _valuesOf(form));
+  }
+
+  /// Reports what is in the form once the reader pauses, so a search box
+  /// narrows its page without being submitted.
+  void _changed(FormBlock form, String name, String text) {
+    _values[name] = text;
+    final report = widget.onFormChange;
+    if (report == null) return;
+    _typing?.cancel();
+    _typing = Timer(searchTypingPause, () => report(form, _valuesOf(form)));
+  }
+
+  Map<String, String> _valuesOf(FormBlock form) {
     final values = <String, String>{};
     for (final field in form.fields) {
       switch (field) {
@@ -851,7 +916,7 @@ class _MewsFormState extends State<_MewsForm> {
           break;
       }
     }
-    widget.onFormSubmit(form, values);
+    return values;
   }
 
   void _reset(FormBlock form) {

app/lib/presentation/reader/reader_screen.dart

@@ -242,7 +242,10 @@ class _ReaderScreenState extends State<ReaderScreen> {
         url: page.url,
       );
       return MewsPageView(
-        key: ValueKey(page.url),
+        // Our own pages carry their view state in the address, so the key
+        // leaves the query out: narrowing a list as the reader types rebuilds
+        // the page without taking the search box out from under them.
+        key: ValueKey(_viewKey(page.url)),
         page: page,
         ownPage: MewsUrls.isInternal(page.url),
         palette: palette,
@@ -252,12 +255,23 @@ class _ReaderScreenState extends State<ReaderScreen> {
         onLinkTap: (url) => deps.browser.open(url),
         onFormSubmit: (form, values) =>
             deps.browser.submitForm(form.action, form.method, values),
+        onFormChange: (form, values) =>
+            deps.browser.narrowWhileTyping(form.action, values),
         fetchImageBytes: deps.browser.fetchImageBytes,
         scrollController: _scroll,
       );
     });
   }
 
+  /// What identifies the open page to the widget tree. One of our own pages
+  /// is identified by its path alone, so a change to its query string, such
+  /// as a search being narrowed, rebuilds the page in place instead of
+  /// building a new one.
+  String _viewKey(String url) {
+    if (!MewsUrls.isInternal(url)) return url;
+    return Uri.parse(url).path;
+  }
+
   /// Renders a neighboring page for the swipe peek, non-interactive: it is a
   /// look ahead, and committing the navigation brings the real thing.
   Widget? _neighborView(MewsPage? page) {