perf(widgets): split VirtualSelectableTextView lines only when text changes (#1532)

lib/core/widgets/virtual_selectable_text_view.dart

@@ -5,7 +5,7 @@ import 'package:flutter/material.dart' as material;
 /// the entire block works seamlessly.
 /// If line count > [threshold], virtualizes lines using `ListView.builder` to
 /// ensure 60 FPS scrolling and rendering without UI jank.
-class VirtualSelectableTextView extends material.StatelessWidget {
+class VirtualSelectableTextView extends material.StatefulWidget {
   const VirtualSelectableTextView({
     super.key,
     required this.text,
@@ -19,26 +19,46 @@ class VirtualSelectableTextView extends material.StatelessWidget {
   final int threshold;
   final material.EdgeInsets padding;
 
+  @override
+  material.State<VirtualSelectableTextView> createState() =>
+      _VirtualSelectableTextViewState();
+}
+
+class _VirtualSelectableTextViewState
+    extends material.State<VirtualSelectableTextView> {
+  /// `widget.text` split on `\n`. Computed when the text changes, not in
+  /// `build`: resize, theme and focus rebuilds must not re-split a text of
+  /// thousands of lines on every frame (#1532).
+  late List<String> _lines = widget.text.split('\n');
+
+  @override
+  void didUpdateWidget(covariant VirtualSelectableTextView oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (oldWidget.text != widget.text) {
+      _lines = widget.text.split('\n');
+    }
+  }
+
   @override
   material.Widget build(material.BuildContext context) {
-    final lines = text.split('\n');
-    if (lines.length <= threshold) {
+    final lines = _lines;
+    if (lines.length <= widget.threshold) {
       return material.SingleChildScrollView(
-        padding: padding,
+        padding: widget.padding,
         child: material.SelectableText(
-          text,
-          style: style,
+          widget.text,
+          style: widget.style,
         ),
       );
     }
 
     return material.ListView.builder(
-      padding: padding,
+      padding: widget.padding,
       itemCount: lines.length,
       itemBuilder: (context, index) {
         return material.SelectableText(
           lines[index],
-          style: style,
+          style: widget.style,
         );
       },
     );

test/core/widgets/virtual_selectable_text_view_test.dart

@@ -40,5 +40,65 @@ void main() {
       expect(find.text('Virtual Line 0'), findsOneWidget);
       expect(find.text('Virtual Line 1'), findsOneWidget);
     });
+
+    testWidgets('a new text re-splits the lines (#1532)', (tester) async {
+      final large = List.generate(50, (i) => 'Virtual Line $i').join('\n');
+      var text = 'a\nb';
+      late void Function(void Function()) rebuild;
+
+      await tester.pumpWidget(
+        material.MaterialApp(
+          home: material.Scaffold(
+            body: material.StatefulBuilder(
+              builder: (context, setState) {
+                rebuild = setState;
+                return VirtualSelectableTextView(text: text, threshold: 10);
+              },
+            ),
+          ),
+        ),
+      );
+      expect(find.byType(material.ListView), findsNothing);
+
+      rebuild(() => text = large);
+      await tester.pump();
+      expect(find.byType(material.ListView), findsOneWidget);
+      expect(find.text('Virtual Line 0'), findsOneWidget);
+
+      rebuild(() => text = 'only\none');
+      await tester.pump();
+      expect(find.byType(material.ListView), findsNothing);
+      expect(find.text('only\none'), findsOneWidget);
+    });
+
+    testWidgets('a rebuild with the same text keeps the same lines (#1532)',
+        (tester) async {
+      final text = List.generate(50, (i) => 'Virtual Line $i').join('\n');
+      var style = const material.TextStyle(fontSize: 12);
+      late void Function(void Function()) rebuild;
+
+      await tester.pumpWidget(
+        material.MaterialApp(
+          home: material.Scaffold(
+            body: material.StatefulBuilder(
+              builder: (context, setState) {
+                rebuild = setState;
+                return VirtualSelectableTextView(
+                  text: text,
+                  style: style,
+                  threshold: 10,
+                );
+              },
+            ),
+          ),
+        ),
+      );
+      expect(find.text('Virtual Line 0'), findsOneWidget);
+
+      rebuild(() => style = const material.TextStyle(fontSize: 14));
+      await tester.pump();
+      expect(find.byType(material.ListView), findsOneWidget);
+      expect(find.text('Virtual Line 0'), findsOneWidget);
+    });
   });
 }