[edit: re-read your post; you're well aware what handoff is, so sorry for the overly vivid explanation]

That's 'cell planning'. Roaming is different:

Let's say you're moving from your bedroom to the kitchen. The bedroom AP's signal suddenly drops. Your smartphone needs to find the kitchen AP, re-connect, and re-transmit all the packets that might have been lost.

Mobile networks assist the smartphone: it tells the smartphone that the kitchen AP will soon be in range. It already allocates and configures resources at the target AP. Then the handover is more seamless.

In theory, WiFi can do this too. But last time I checked, it seemed a vendor-specific mess.