What is the use/meaning of sizeUndefined and when it is useful ?

It removes the CSS size for that element, making its size grow based on its content instead of having a fixed size

Roughly speaking, an element can be sized in three ways (both horizontally and vertically):

  1. it can have a fixed size, like 150px or 40em
  2. it can have a size relative to a parent element (or the viewport, or something else), like 50%
  3. it can be undefined, when it will be as big as its content requires, like a span that grows based on how much text is inside it

A VerticalLayout is by default undefined height, it will grow higher when you add components inside it.

Sounds like a good explanation for the docs :smirk:

The docs should probably have a more detailed description because there are a handful of complications to that story.