Clip behavior

You have 4 choices for migrating your code:

1. Leave your code as is if your content does not need to be clipped (for example, none of the widgets' children expand outside their parent's boundary). This will likely have a positive impact on your app's overall performance.
2. Add clipBehavior: Clip.hardEdge if you need clipping, and clipping without anti-alias is good enough for your (and your clients') eyes. This is the common case when you clip rectangles or shapes with very small curved areas (such as the corners of rounded rectangles).
3. Add clipBehavior: Clip.antiAlias if you need anti-aliased clipping. This gives you smoother edges at a slightly higher cost. This is the common case when dealing with circles and arcs.
4. Add clip.antiAliasWithSaveLayer if you want the exact same behavior as before (May 2018). Be aware that it's very costly in performance. This is likely to be only rarely needed. One case where you might need this is if you have an image overlaid on a very different background color. In these cases, consider whether you can avoid overlapping multiple colors in one spot (for example, by having the background color only present where the image is absent).

For the Stack widget specifically, if you previously used overflow: Overflow.visible, replace it with clipBehavior: Clip.none.

For the ListWheelViewport widget, if you previously specified clipToSize, replace it with the corresponding clipBehavior: Clip.none for clipToSize = false and Clip.hardEdge for clipToSize = true.

Code before migration:

    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: Center(
          child: Stack(
            overflow: Overflow.visible,
            children: const <Widget>[
              SizedBox(
                width: 100,
                height: 100,
              ),
            ],
          ),
        ),
      ),
    );

Code after migration:

    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: Center(
          child: Stack(
            clipBehavior: Clip.none,
            children: const <Widget>[
              SizedBox(
                width: 100.0,
                height: 100.0,
              ),
            ],
          ),
        ),
      ),
    );