Where do USB ports on laptops typically source their power when the laptop is charging?
If the voltage at the bus connected to the battery is higher than the battery voltage (given by the State of Charge of the battery), current will flow into the battery, charging it. 

In this case, current can obviously not flow from the battery at the same time. Thus, the energy to supply the laptop has to come from the power supply brick attached to it.

This may change fast, as switch mode power supplies can regulate voltages fast. It may go from charging to discharging the battery depending on cpu load, if the PSU is not able to supply the maximal consumption by the laptop, for instance. When battery is discharging, the voltage of the battery is higher than the bus, even if there's other supplies connected the same bus.