1) just as designed and has almost no chances to be changed in future because you are the first who requires this feature and because we have to much customers which waits for other fixes, changes and so one..
2) not sure that understand you.
Just clarify number 2):
Action: user does not drag the horizontal scrollbar, instead moves horizontal scrollbar by click mouse on the scrollbar track.
Expect result: suppuse there are totally 30 columns, first 4 are frozen columns, before the user performances above action, user can see totally 10 columns, after the action, 6 columns should be moved to left, so the 11th column will be the first column after 4 frozen column, so on.
Currently, I don't know the exact result, but looks like, every click, it moves above 10 columns (same width as table?).
Try that on horizontal scrollbar of other application like Excel, you may get idea.
we do not contol the scroll in this case - browser shifts it to new position :)