Fix RenderCustomPaint intrinsics (#72103)

Make RenderCustomPaint consult its preferred size for intrinsics when it has no child. Currently, it returns 0 for its intrinsics, even if it has a non-zero preferred size.

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

@@ -487,6 +487,34 @@ class RenderCustomPaint extends RenderProxyBox {
   /// to change in the next frame.
   bool willChange;
 
+  @override
+  double computeMinIntrinsicWidth(double height) {
+    if (child == null)
+      return preferredSize.width.isFinite ? preferredSize.width : 0;
+    return super.computeMinIntrinsicWidth(height);
+  }
+
+  @override
+  double computeMaxIntrinsicWidth(double height) {
+    if (child == null)
+      return preferredSize.width.isFinite ? preferredSize.width : 0;
+    return super.computeMaxIntrinsicWidth(height);
+  }
+
+  @override
+  double computeMinIntrinsicHeight(double width) {
+    if (child == null)
+      return preferredSize.height.isFinite ? preferredSize.height : 0;
+    return super.computeMinIntrinsicHeight(width);
+  }
+
+  @override
+  double computeMaxIntrinsicHeight(double width) {
+    if (child == null)
+      return preferredSize.height.isFinite ? preferredSize.height : 0;
+    return super.computeMaxIntrinsicHeight(width);
+  }
+
   @override
   void attach(PipelineOwner owner) {
     super.attach(owner);

packages/flutter/test/widgets/custom_paint_test.dart

@@ -198,4 +198,20 @@ void main() {
     expect(() => CustomPaint(isComplex: true), throwsAssertionError);
     expect(() => CustomPaint(willChange: true), throwsAssertionError);
   });
+
+  test('RenderCustomPaint consults preferred size for intrinsics when it has no child', () {
+    final RenderCustomPaint inner = RenderCustomPaint(preferredSize: const Size(20, 30));
+    expect(inner.getMinIntrinsicWidth(double.infinity), 20);
+    expect(inner.getMaxIntrinsicWidth(double.infinity), 20);
+    expect(inner.getMinIntrinsicHeight(double.infinity), 30);
+    expect(inner.getMaxIntrinsicHeight(double.infinity), 30);
+  });
+
+  test('RenderCustomPaint does not return infinity for its intrinsics', () {
+    final RenderCustomPaint inner = RenderCustomPaint(preferredSize: const Size.square(double.infinity));
+    expect(inner.getMinIntrinsicWidth(double.infinity), 0);
+    expect(inner.getMaxIntrinsicWidth(double.infinity), 0);
+    expect(inner.getMinIntrinsicHeight(double.infinity), 0);
+    expect(inner.getMaxIntrinsicHeight(double.infinity), 0);
+  });
 }