For some reason it's not possible to drag that splitter.
This seems to be style-specific, it's incredibly slow in Oxygen but works sort-of OK in QtCurve. I wonder if this is somehow related to the massive slowdowns I experience when opening a log viewer tab, i.e. something is doing busywork in IrcView. Vaguely reminds me of the old problems with resize+scrollbar-toggle loops we used to have.
I can't reproduce this anymore, seems it got fixed in the styles or Qt somewhere.