The first issue is normal - as more text is displayed in the window, the scrollbar will decrease in size to indicate the amount of text in the window. This is actually a windows feature, so it's not something I can change.
The second issue is by design - if there are thousands of lines in your window, and you resize the window at some point, mIRC only updates and resizes them if you scroll back to view them, and this in turn updates the scrollbar size.
The third issue is also by design - when the number of lines in your window exceed the window buffer size that you have set in the Options/Other dialog, mIRC removes lines from the top of your window buffer. I could make mIRC remove whole lines, however it has used the current method for many years.
