Sometimes, I've noticed this problem, but is usually caused when a pge is slow to load. Intermittent slow connections, or whatever, so the page is still loading, just appears to be broken.
A quick technical explanation is that the window resizes itself once it calculates how large the page is. The forum really is a software window INSIDE a software window. Not 100% ideal, but a neccessity the way the site is built. It's complex
How many other people experience this frequently? Again, its very rare, and I use the same browser version. I've also noticed it if you sometimes click on a link before the page loads. I am still trying to see if that is the culprit or not.... but again, 50% is too frequent.
A hint to reload in the future. Instead of pressing reload or CTRL-R, which will reload the index page, right click inside the thread, or forum index or wherever you are inside the message board (not right clicking on a link) and then click reload/refresh. That will only reload the message forum window, and essentially resize the window if needed.