My thought was that it's a DC max-width canvass issue, and that mIRC might consider truncating canvass widths to something much smaller, like 8192 px wide for non-picture windows.

That is also possible. There are quite a few arbitrary limits throughout the code and across many features that, at the time of coding years ago, seemed reasonable but may now be too limiting or small. I will need to look through all of the display-related routines to track this down though.