Blake,
I am ok of customizing this. Matt's suggestion did it.

I am talking about a default setting for most users. As of now, by default, mixer is docked.
Once you "undock" it and stretch it... you would get that black empty space (default panel height 50). And if a default Panel Height size is set to 140, no matter how you stretch it, it would do it dynamically, without black window.

All I am saying, I think 140 panel height should be a default and others "optional".