I initially thought this was not a bug but now I'm not so sure. I'm using a 4 x 3 19" monitor at 1024 x 768 with a new Athlon Phenom 6 core, 4 gigs ram, 1TB HD running Win 7 64 bit using an on board ATI HD video chip on a Gigabit mobo. I opened a 32 bar song. That exactly fills up my screen with the 33rd bar for the ending partially cut off at the bottom. The screen would not scroll. The song has 4 chorus repeats in it so I unfolded it to one long chorus. The program automatically shrunk the size of the chord cells by half so it now displays 64 bars with the next line starting at bar 65 partially cut off at the very bottom. The screen would not scroll and when I let the song play, when it got to bar 64 the display somehow wrapped itself so the cursor was at the top but the chord grid was partially overlaid on top of the previous 64 bars and yeah you could say it looked "smeary" but I would be more precise and say I could see a bit of the chord grid underneath the current one but whatever we call it it was definitely not right. I opened another song that I knew was over 100 bars and the same thing happened but I could swear that when I played that song the other day everything was fine. This sounds like what you're describing and I have never seen that before.
I'm using files in the Biab Real Book that I downloaded from somewhere so there could be things in the file that's messing this up. I closed Biab and upon reopening with the same unfolded song, the screen now scrolls properly and so does the second song I tried plus 3 more. At least I did see what you're describing Eddie but whether or not I can consistently reproduce it is another question because it's working fine now. I can't simply say check your set up because I did just see the same thing.
This is my weekend to devote to this anyway since I'm also trying to get my Sonic Cell completely set up so I will test this screen issue while I'm playing with different Sonic Cell patches but instead of staying on one song for a while I'll make sure to test a bunch of them.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.