I've noticed some alignment issues when using Moodle 4.0+ and a basic BOOST theming with "Topics" course format (other themes may experience similar issues).

To reproduce:
Open a forum, h5p, questionnaire type activity (others likely) and expand the window size to a large window (more than 1800px). Note that the forum elements have a different max-width: setting. By contrast, opening a page, assignment, or quiz type activity follows the same max-width size used for the new secondary nav / horizontal toolbar (~830px)

I've wondered why we aren't using the full 100% max-width available and came up with a little CSS override to test out if I could fix the issue. 

.pagelayout-standard #page.drawers .main-inner, 

body.limitedwidth #page.drawers .main-inner,

header#page-header, .secondary-navigation

{max-width: unset!important;} 

It does "fix" the problem with different activities having different max-width's and utilizes all screen space — but may cause other issues / not extensively tested.

Screenshots attached.
Not being fully versed in Moodle CSS hierarchy, who's issue is this to fix? Individual activity groups or BOOST theme?