Unify position-fixed behaviour

[AdaRoseCannon]

A bunch of elements have position:fixed and width:100% because they are position:fixed, width:100% isn't the parents width it's the page width.

There needs to be consistent width logic for the container which also gets applied to all position:fixed elements e.g.

• Toast
• Dialog
• Tabs

[AdaRoseCannon]

Perhaps a .fixed class for any element that is fixed that manages the size of fixed elements and uses the same logic and is in the same file as the container logic.

What do you think @lauramorinigo @PicchiKevin