Say a character with 20 strength has the jump spell cast upon them. On the other side of a 40 foot chasm is an enemy standing next to the ledge. The character takes a 10 foot run up and leaps over the chasm, planning to land next to the enemy. Unbeknownst to them however, the enemy is a spell caster who has cast antimagic field previously.
The characters strength score, jump spell and the 10 foot run up they took means they can jump 60 feet. However, they will find themselves in the area of effect of an antimagic field when they are still 10 feet from the other side of the ledge.
So what happens? Do they immediately stop all forward movement when they enter the field and drop down into the chasm because the jump spell has been suppressed? Or does the antimagic field have no effect because their jump distance was determined when they took off (ie. because physics)?