What do I need to switch power automatically from utility to solar offgrid inverter with no downtime at all during the transfer?
As mentioned in comments, typical inverter switch times are fine for most devices.

The only way I know of to literally have NO downtime is dual-conversion, where your inverter is supplying the AC power 100% of the time, and your grid power is converted to DC and fed into the battery / DC input of the inverter.

That has built-in inefficiencies (thus, wasted power), but is "zero-switch" time as there's never a switch on the inverted power, you just go from float-charging the batteries from AC &amp; solar to drawing down the batteries/solar.