Please note: This was written for IPB 2.1.x but should work correctly in other versions as well as other software, scripts and web pages that use CSS.
Ever noticed how when you shrink the window with your forum in it how it all jumps out of place.
Go to your AdminCP and open your CSS, move down a little until you see the
body { tag. In there add
Save that and refresh your forum page, now when you shrink the window the board should stop shrinking when it hits 1000 pixels. You can set this to anything you want I just like 1000.