This kept me busy for an hour or two, figuring out how to animate a width property on a widget from a relative starting position to an absolute end position.
My expectation was that I could simply add an absolute keyframe add the end of the track, and either not specify any other keyframe or mark it as relative.
Not the case. After you have added an absolute track spanning over X time with a keyframe at the end, you need to add a second track, a relative one. Make sure to drag the end of the track so it uses the entire width of the absolute track it must blend with.
So what now, done? Do you need to add a keyframe there? how in the world does it blend? The expectation was that I had to open up the curve editor. Whatever this empty screen does, it does not blend
Back to the keyframe panel. Apparently you can drag and drop the relative track on top of the absolute track as long as the relative one has no keyframes. Finally, I can barely see some sort of curve. Except the values are nothing like I specified.
So that resulted in garbage values. Ctrl - Z and search the forums. Turns out the way to blend is to drag a super tiny barely visible rectangle left or right from one of the ends on both the absolute track and the relative track on the keyframe panel. The higher the curve value the more influence the track gets.
Seriously. Who is this panel build for? Makes me wish we could just enter CSS or a script to animate because this doesn’t work for programmers, designers or anyone else. Can we make that tiny triangle 2 pixels instead of 5 so that no one can use it? Maybe make it white on white? 
Some points for improvement:
- Add a guide somewhere, didn’t find one.
- Increase contrast, no blue on cyan or white on grey please.
- Increase interactive widget size so they are easier to click.
- Make the curve editor useful, I can barely see the blue curve on that relative track, we need a proper panel for it.
- Dont let interactive widgets overlap like this, there’s the triangle + the track length + the keyframe + overlapping vertical lines and whatnot on the same pixels.