Not sure of the exact cause of this, but I've been having this problem for a long time (since v6.*, it didn't occur in 5.91), I just hadn't had time to put it here. I am using Windows XP Home and currently 6.12. It's a bit hard to explain, but I'll try...

mIRC keeps a buffer of all messages/joins/etc in the channel window (we all know this), but when the buffer starts to grow larger the scrollbar from within that window, be it channel, query, dcc chat, status, etc., doesn't 'update' properly.
Say the scrollbar is 3mm big, when you grab it, scroll completely up to the top of the window then back down it again, it 'grows' to about 3cm (for example), and vise versa, it grows and shrinks randomly.
Using my scrollwheel on my house causes the window to 'shake' up and down. Pressing "Page UP" only makes it jump like 6 lines. Both these problems can be fixed by grabbing the scroller and moving it up to the top and then back to the bottom, and all returns to normal.

I hope I explained it alright... What's up with this?

yes ive also noticed this affect too, one thing to mention is do u have an up to date mouse driver? are u using one from the mouse company or one from microsoft, sometimes that may also make a difference. i only experiance this in mirc and not in any other programs, im kinda curious about it also.

Logitech mouse drivers, it's also affected by the keyboard as well (page up + down buttons).

yup, its some sort of screen buffer/scrollbar inconsistancy
and its frikken annoying when you're trying to read a large log

Don't really get you. However, I've tried grabbing the scroll bar, scroll fully up, halfway up, up and down. Whatever, the size remains the same. Using a brand-less scroll mouse which says PS/2 Compatible Mouse under device manager. v6.12 xppro sp1

what visual display style are you using? i've noticed my logitech gets cranky unless i use noframe

Hi

I have the same Page Up and Page Down buttons problem here.

It sometimes doesn't scoll up or down 1 page, it just seems to scoll like half of the page or something. I noticed that it usually happens in the windows of large channels where there's too much activity.

I'm using no frames here. Maybe that explains why I'm not affected?

No frame, Light switchbar font selected, Hide bar separators selected, Switch buttons selected, Black border colour, Border size 1.

I turned on noframe and it doesn't seem to do it anymore... Perhaps it was that, I used "Thin Frame" previously.

been like that ever since we could make the frame choice for me and others. seems logitech drivers get cranky easily

I got the same effect here on Win2k ... using a microsoft (intelli explorer) mouse and the standard microsoft mouse drivers (delivered with windows .. ).

I had the same effect with my previous noname mouse (using microsoft drivers too..)

Nope, still has the same effect no matter what frame type I use.

I hate posted this bug for 6.03 already complete with screenshots and the likes...

not seen it in 6.12 afaik... not happend to me anyways

with 6.03 it happend sometimes... but the reason why was not clear. it wasn't with very busy channels or anything... just a few windows that were affected from time to time.

text that spread more lines was just cut in half (first line of the buffer)

really weird bug it was