With Navigator 7.3rc1 the stack selector button is unavailable. What's shown in its place is a crosshair icon, which corresponds to the whichTarget button. A little work with the PI shows that the whichStack button has managed to have a width of 4 pixels instead of 24. Fixing the width of the whichStack button (and setting the visible of the whichTarget button to false) brings the stack selector back into operation.
This was intentional, although you're not the only one the change confused. If you:
Click the WhichTarget button -- the crosshairs appear, and you can click any stack to display it in Navigator. Note that if Show IDE Stacks is not selected, clicking an IDE stack with the crosshairs will do nothing.
Click and hold on the WhichTarget button -- the WhichStack button will drop down as usual. While continuing to hold the mouse down, drag to whichever menu item you like and release to select it.
Click for > 10 ticks and < 30 ticks -- the WhichStack button drops down. Mouse over whichever menu item you like and click to select it.
With Navigator 7.3rc1 the stack selector button is unavailable. What's shown in its place is a crosshair icon, which corresponds to the whichTarget button. A little work with the PI shows that the whichStack button has managed to have a width of 4 pixels instead of 24. Fixing the width of the whichStack button (and setting the visible of the whichTarget button to false) brings the stack selector back into operation.