You can test this for yourself.
After navigating to the invoice layout, use new window from the window menu to open a second window to the same layout. In the first window start editing values in a portal row. Then switch to the other window and try editing one of the other portal row records for the same invoice. I believe that you'll get an error message telling you that the record is locked in the other window, but test and find out for yourself to be sure.
Thanks I will test and confirm with you Phil!