You can also enable this in code:

Highlight repaints

This option draws a border around all render boxes
that changes color every time that box repaints.

This rotating rainbow of colors is useful for finding parts of your app
that are repainting too often and potentially harming performance.

For example, one small animation could be causing an entire page
to repaint on every frame.
Wrapping the animation in a RepaintBoundary widget limits
the repainting to just the animation.

Here the progress indicator causes its container to repaint:

Wrapping the progress indicator in a RepaintBoundary causes
only that section of the screen to repaint:

RepaintBoundary widgets have tradeoffs. They can help with performance,
but they also have an overhead of creating a new canvas,
which uses additional memory.

You can also enable this option in code:

Highlight oversized images