Remove NavigationToolbar condition that leading widget cannot be larger than...

Remove NavigationToolbar condition that leading widget cannot be larger than 1/3 the total space available. (#112548)

packages/flutter/lib/src/widgets/navigation_toolbar.dart

@@ -106,7 +106,7 @@ class _ToolbarLayout extends MultiChildLayoutDelegate {
 
     if (hasChild(_ToolbarSlot.leading)) {
       final BoxConstraints constraints = BoxConstraints(
-        maxWidth: size.width / 3.0, // The leading widget shouldn't take up more than 1/3 of the space.
+        maxWidth: size.width,
         minHeight: size.height, // The height should be exactly the height of the bar.
         maxHeight: size.height,
       );

packages/flutter/test/material/app_bar_test.dart

@@ -3680,4 +3680,34 @@ void main() {
     await tester.pumpWidget(buildApp());
     expect(tester.getTopLeft(title).dx, 16.0);
   });
+
+  testWidgets('AppBar leading widget can take up arbitrary space', (WidgetTester tester) async {
+    final Key leadingKey = UniqueKey();
+    final Key titleKey = UniqueKey();
+    late double leadingWidth;
+
+    Widget buildApp() {
+      return MaterialApp(
+        home: LayoutBuilder(
+          builder: (BuildContext context, BoxConstraints constraints) {
+            leadingWidth = constraints.maxWidth / 2;
+            return Scaffold(
+              appBar: AppBar(
+                leading: Container(
+                  key: leadingKey,
+                  width: leadingWidth,
+                ),
+                leadingWidth: leadingWidth,
+                title: Text('Title', key: titleKey),
+              ),
+            );
+          }
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp());
+    expect(tester.getTopLeft(find.byKey(titleKey)).dx, leadingWidth + 16.0);
+    expect(tester.getSize(find.byKey(leadingKey)).width, leadingWidth);
+  });
 }