As of now, the omnibox text field supports cursor placement, but not selection.  This means that if there's a long string typed into the field, there isn't a direct way to move the cursor to an off-screen location.

This could be alleviated by (partially) supporting selection, in that a click and drag on text could scroll the field.

I started prototyping this, and found that in the button-down case, our hit test code no longer reports reticle locations outside the element bounds.  Therefore, we can't support dragging the reticle off the ends of the field to increase the selection.  We'll have to return to older behavior where we'd report off-element coordinates, and rely on elements to filter properly in this case.