zero-sized RenderConstraintsTransformBox respects clipBehavior (#147349)

Fixes flutter/flutter#146840

Author: LongCatIsLooong

packages/flutter/lib/src/rendering/shifted_box.dart

@@ -896,9 +896,7 @@ class RenderConstraintsTransformBox extends RenderAligningShiftedBox with DebugO
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    // There's no point in drawing the child if we're empty, or there is no
-    // child.
-    if (child == null || size.isEmpty) {
+    if (child == null) {
       return;
     }
 
@@ -919,6 +917,9 @@ class RenderConstraintsTransformBox extends RenderAligningShiftedBox with DebugO
 
     // Display the overflow indicator if clipBehavior is Clip.none.
     assert(() {
+      if (size.isEmpty) {
+        return true;
+      }
       switch (clipBehavior) {
         case Clip.none:
           paintOverflowIndicator(context, offset, _overflowContainerRect, _overflowChildRect);

packages/flutter/test/rendering/box_test.dart

@@ -503,6 +503,23 @@ void main() {
       // At least 2 lines.
       expect(constrainedHeight, greaterThanOrEqualTo(2 * unconstrainedHeight));
     });
+
+    test('paints even when its size is empty', () {
+      // Regression test for https://github.com/flutter/flutter/issues/146840.
+      final RenderParagraph child = RenderParagraph(
+        const TextSpan(text: ''),
+        textDirection: TextDirection.ltr,
+      );
+      final RenderConstraintsTransformBox box = RenderConstraintsTransformBox(
+        alignment: Alignment.center,
+        textDirection: TextDirection.ltr,
+        constraintsTransform: (BoxConstraints constraints) => constraints.copyWith(maxWidth: double.infinity),
+        child: child,
+      );
+
+      layout(box, constraints: BoxConstraints.tight(Size.zero), phase: EnginePhase.paint);
+      expect(box, paints..paragraph());
+    });
   });
 
   test ('getMinIntrinsicWidth error handling', () {