DEV: Some select-box headers don't need a class (PR #12992)

The dropdown-select-box-header component adds a btn-default class to the button it creates… but not all the components that extend dropdown-select-box-header should have that class.

This causes some minor annoyances in themes, for example if I do:

.btn-default {
  background: salmon;
}

:+1: most of the time I want this class’s styles to apply to the dropdown-select-box-header… for example here:

:-1: but then I have to undo it here (and in a couple other places):

Screen Shot 2021-05-07 at 10 59 08 PM

.btn-default.period-chooser-header {
  background: transparent;
}

So what I’m adding in this PR is the ability to turn off that btn-default class selectively.

GitHub

DO you think this is necessary to merge now? or we wait after release ?

@jjaffeux can wait until after release