vertical ruler

What about it being on only when you mouseover a the overview/navbar?

Mouseover ruler = create div; mouse move & mouse exit = move/hide div ?

I like Ian's suggestion of how to implement this. It is spot on.

It would also play nicely with rubber-band zooming requested by ticket #59 (and #42 and #21), if you implement it by dragging to select a zoom region on the coordinates bar. So it goes like this: You put your cursor on the coordinates bar to start select a zoom region. You can see where the region will start because a vertical bar pops up, before you even click and drag. Now, you can click and drag to select.

In the UCSC browser, I am frequently annoyed that when I start selecting a rubber-band zoom region, I "miss" the start point and have to drag out-of-view to cancel selection and then re-start. The region is not highlighted until you start dragging. Which means that if your first click was in the wrong spot, you don't get to know it until you already started the selection! If you get a vertical bar that tells you where the cursor is with respect to the tracks below, you don't "miss" as badly.

Incidentally, in the UCSC browser, I am frequently forced to use their rubber-band zoom as a hack to get this vertical ruler feature! So believe me, it would be nice to have. Vertical gridlines aren't enough -- you need a line that follows the cursor around.