Render tree

If you are trying to debug a layout issue, then the Widgets layer’s
tree might be insufficiently detailed. In that case, you can dump the
rendering tree by calling debugDumpRenderTree().
As with debugDumpApp(), you can call this more or less any time
except during a layout or paint phase. As a general rule,
calling it from a frame callback
or an event handler is the best solution.

To call debugDumpRenderTree(), you need to add import
'package:flutter/rendering.dart'; to your source file.

The output for the previous tiny example would look something like
the following:

This is the output of the root RenderObject object’s
toStringDeep() function.

When debugging layout issues, the key fields to look at are the
size and constraints fields. The constraints flow down the tree,
and the sizes flow back up.

RenderPositionedBox to be the size of the screen, with
constraints of