ui: slide up, down, left and right use the full amount of space now

[skiesel]

…rather than only delta increments

Fixes #239

[eaburns]

Remove the "could be multiplication" comments. You got it right. There is no guarantee that they will all remain the same.

Other than that, I just want to think if there is a way to test it.

[eaburns]

Checkout window_test.go. It has a bunch of tests for UI interactions like clicking and dragging etc. I think you can test these in there.

Test vertical sliding by making a window that is 3.5*minHeight. It should fit 3 frames with a tiny bit of slop. Then send a 1-click to the middle frame (run the tests with -race — it's so easy to get a race condition here if you don't add wait(w) in various places, like between interacting with a frame and reading a frame's center point, center(sheet0)). The test should check that the clicked frame (wait(w) between the click and checking this) bounding box grew by some amount, but not by as much as minHeight. Double check that it was actually hitting both vertical cases using go test -coverprofile=c && go tool cover -html=c.

Phew.

For the horizontal case — well, let's get a test working for the vertical case first, while I think about this.

[eaburns]

Ping.