What is the purpose of this purple dashed line area?

Question

What does this purple area represent?

Answer 1

It shows the available space where the element can be expanded.

Example:

This is an example with just one character. It is possible to see the dashed area after the text. It means that it is the area where the text can possibly be expanded.

Now let's add more characters in our example. It can be seen that the length of the purple dashed line area is decreased:

Code snippet

It is possible to run the following code snippet and open dev tools to see that purple dashed area:

*, html, body {
    box-sizing: border-box;
    margin: 0;
}

div {
    position: relative;
    background-color: lightgreen;
}

button {
  display: flex;
  width: 100px;
}

<div>
    <button>1</button>
</div>