I Cannot seem to duplicate the problem. Can you take a screen shot of the problem and attach to this thread?
I don't think attaching a screenshot will help much...but here ya go (some notes added to it)
Here are the exact steps taken:
1. Open thread.
2. Click post #1 'Reply with quote'
Hmmm, quite strangely, the problem is also occurring as I type this message... making it rather difficult
3. Scroll the mousewheel down past the [ / QUOTE ] tag & left-click to start typing. Upon left-click or mouse movement, the reply box scrolls upwards.
4. Sometimes, before the pointer has got to where I want to click to type, the reply box has already scolled up..
5. Once the scrolling starts, mouse movement seems to accelerate it and the scroll slider cannot be clicked & dragged until it has reached the top. (on the original post only, this one can be clicked anywhere along it's height..so maybe it has something to do with the length of quoted text??)
EDIT :
Using the 'Go Advanced' function sometimes seems to cause the scroll bar to blink, when not fully at either extent, when crossing borders of a box, so it may not just be a quirk solely in quick-reply...