Recently the sidebar was widened a bit, but if your window isn't wide enough for the full width that just means you lose some sidebar -- the main content area didn't change. That's good; we don't want to disrupt the usability of the primary content.
However, I've noticed that some moderation pages, pages which do not have the sidebar, have also become wider, and that does affect usability. Here are some places I've noticed this (no screen shots because mod tools):
on the moderator dashboard: the last column is now wholly out of view for me
on the moderator users page: recent messages are cut off for me and require horizontal scrolling to read
on the per-user comment history: posting dates are now out of view for me (those are important!)
I'm assuming that this got carried along with the widening of the topbar and site banner; I see that the "ask question" link is now out of view for me too and wasn't previously.
If I have to scroll to ask a question I'll live, but having to constantly scroll back and forth to do common moderator tasks is frustrating. No, "make the browser window bigger" isn't an option. It's not tiny; all this is stuff that used to provide a smoother user experience for me at this window size (around 1000px).
I don't think widening the moderator pages was an intended consequence of the design change, so I'm tagging this as a bug. This doesn't happen on all pages; for example, the moderator analytics page is not over-wide.