Apple - Taking a screenshot of a tooltip

You can do this using the screencapture CLI:

screencapture -T 5 -C ~/Desktop/screencapture.png

^{Define a delay using -T <seconds>. Use -C to capture the cursor too.}

Add -i to capture interactively. This uses the normal capture region selection available from ⌘⇧4, however you can combine this with -T to add a delay so that it captures the tooltip in the region.

The trick is simple, you have to keep press (⌘+SHIFT) first. Then hover mouse to make tool tip appear and press (4).

You can do this with the Capture > Timed Screen option in the Grab utility. After starting the timer, you'll have 10 seconds to get back to what you want a shot of (including getting the tooltip to reappear). Also, you can use the Grab application's preferences to control whether (& how) the mouse pointer appears in the shot. What it doesn't let you do is choose which region to take a shot of, it just grabs the entire screen; you'll have to crop it appropriately yourself.