Idea: adaptive circle positioning

[anchpop]

We already have the ability to have the position of the circle be dependent on the initial tap. But when typing blind, it's easy to lose track of where the circle is and end up with your finger in the wrong place.

So what if the circle could move as you type? Here's what I have in mind. Basically, we always try to have our finger go through the center of the circle:

But now imagine the case where the user gets a bit confused about where the center of the circle is, and does this:

Here is my proposal for how to recenter the circle. We draw a line using the inclination of the last time the user's finger left the circle:

When the user's finger passes this line, that point becomes the new center:

This idea should probably be combined with #440, to avoid accidental recenterings. Also, it should only apply if the line enters "in a straight line" through the sectors adjacent to the sector it left from at the last iteration. This is to prevent odd situations like this one: