RenderEditableParagraph width looks as if it was:
'parent_width' + _kCursorGap + _kCursorWidth
but not textPainter.size.width + _kCursorGap + _kCursorWidth
EditableTextWidget becomes horizontally scrollable:

Line break looks like:

But comments say:
// Editable text does not support line wrap.
bool get allowLineWrap => false;