I think it is a bug in the CSS but I am not sure, perhaps I have made something wrong.
When I put the forum into a frameset like this

<frameset rows="175,*" border="0" frameborder="0" framespacing="0">
<frame src="01.html" name="navi" NORESIZE>
<frame src="../forum/" name="cont">
</frameset>

there is a horizontal scrollbar in the frame with the forum if there is a vertical scrollbar. In minibb 1.7 this problem didn't occurred.
This thing happens only in IE, not in Firefox. So I think something in the CSS is interpreted in a different way.

Open bb_default_style.css and look for width: 770px; (2x).
Replace it with width: 95%; and your miniBB looks like 1.7 again.

_Marco

You did not understand my problem.
But I already found the solution. It's not a bug in miniBB, it's in IE6.
You can search the web for the "horizontal scrollbar in frames"-problem. There are many sites about it, and different kinds of solutions.
This problem didn't appeared in 1.7 because 1.7 used a doctype, which caused quirks-Mode in IE and the scrollbar exists only in standards-mode.
I solved the problem by adding html { overflow-x:hidden; } to the CSS. Now the CSS isn't valid anymore. There are also other solutions, but none is perfect.

Okidoki, I didn't know that there was more to is. I just know this difference in bb_default_style.css in 1.7 and 2.0.
Have you tried 1.7 with width: 770px; ?

_Marco

What do you mean by just this difference? The CSS of 1.7 and 2.0 are completely different.
And as it turned out the problem I had has nothing to do with the CSS.

Yes, it's a browser-bug.