Not much required for repro steps. Only needs comparing.
This (below) is the activity tab. Note the section on the right and how it extends to the user-header’s edge. It be the same for all tabs on the navigation-left.
This is the notifications tab. Please note the same. This is for all tabs under notifications (left nav).
The content does not make full-width use of the right section’s allotted area.
Latest Chrome; Windows 10.