vis: keep tooltip visible
On the right edge of the viewport, most of the cell's tooltip renders off-screen
There are two apparent ways of solving that:
A. old-vis-style: offset tooltip down and to the left, as we have some screen estate to spare them there due to test names
B.clever: depending on a quater render the tooltip relative to the cursor:
|below, to the right | below, to the left |
|------------------------------------------
|above, to the right | above, to the left |