Ads throwing off sidebar?

[MOC]

Adding an ad unit to "Just Below Header", or anywhere in forum "After first thread/post" pushes the sidebar  off-screen, making the site look lopsided and spawning a horizontal scroll bar.  The expectation would be that the side bar would remain fixed, and the site would responsively grow/shrink to accomodate ad units.  Is there a fix for this?  Anything placed in the main content column appears to kick out the sidebar, even though there is plenty of room...