autocomplete: Simplify updating input on topic autocomplete

The point in this autocomplete UI is to choose the entire new value
for the topic input.  So we can do that directly, without the more
complicated logic that parallels what we need in the more
complicated case of autocomplete in the content input.

This is NFC as far as the text value of the input is concerned.  I
believe it's probably NFC entirely, because the remaining aspects of
TextEditingValue -- the selection and the composing region -- are
probably clobbered anyway by the next line's giving focus to the
content input.  (The new test already passes before this change.)

Author: gnprice

lib/widgets/autocomplete.dart

@@ -321,13 +321,8 @@ class TopicAutocomplete extends AutocompleteField<TopicAutocompleteQuery, TopicA
   void _onTapOption(BuildContext context, TopicAutocompleteResult option) {
     final intent = autocompleteIntent();
     if (intent == null) return;
-    final replacementString = option.topic;
-    controller.value = intent.textEditingValue.replaced(
-      TextRange(
-        start: intent.syntaxStart,
-        end: intent.textEditingValue.text.length),
-      replacementString,
-    );
+    assert(intent.syntaxStart == 0);
+    controller.value = TextEditingValue(text: option.topic);
     contentFocusNode.requestFocus();
   }
 

test/flutter_checks.dart

@@ -66,6 +66,12 @@ extension TextChecks on Subject<Text> {
   Subject<TextStyle?> get style => has((t) => t.style, 'style');
 }
 
+extension TextEditingValueChecks on Subject<TextEditingValue> {
+  Subject<String> get text => has((x) => x.text, 'text');
+  Subject<TextSelection> get selection => has((x) => x.selection, 'selection');
+  Subject<TextRange> get composing => has((x) => x.composing, 'composing');
+}
+
 extension TextEditingControllerChecks on Subject<TextEditingController> {
   Subject<String?> get text => has((t) => t.text, 'text');
 }

test/widgets/autocomplete_test.dart

@@ -17,6 +17,7 @@ import 'package:zulip/widgets/message_list.dart';
 
 import '../api/fake_api.dart';
 import '../example_data.dart' as eg;
+import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../model/test_store.dart';
 import '../test_images.dart';
@@ -312,5 +313,36 @@ void main() {
       await tester.pumpAndSettle();
       checkTopicShown(topic2, store, expected: true);
     });
+
+    testWidgets('text selection is reset on choosing an option', (tester) async {
+      // TODO test also that composing region gets reset.
+      //   (Just adding it to the updateEditingValue call below doesn't seem
+      //   to suffice to set it up; the controller value after the pump still
+      //   has empty composing region, so there's nothing to check after tap.)
+
+      final topic = eg.getStreamTopicsEntry(name: 'some topic');
+      final topicInputFinder = await setupToTopicInput(tester, topics: [topic]);
+      final controller = tester.widget<TextField>(topicInputFinder).controller!;
+
+      await tester.enterText(topicInputFinder, 'so');
+      await tester.enterText(topicInputFinder, 'some');
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: 'some',
+        selection: TextSelection(baseOffset: 1, extentOffset: 3)));
+      await tester.pump();
+      check(controller.value)
+        ..text.equals('some')
+        ..selection.equals(
+            const TextSelection(baseOffset: 1, extentOffset: 3));
+
+      await tester.tap(find.text('some topic'));
+      await tester.pump();
+      check(controller.value)
+        ..text.equals('some topic')
+        ..selection.equals(
+            const TextSelection.collapsed(offset: 'some topic'.length));
+
+      await tester.pump(Duration.zero);
+    });
   });
 }