In my experience some spells seem to continue casting and land at which point no other spells are cast until LOS is obtained. Other spells they'll fail to cast and try to get LOS immediately. Also, running out of spell range always guarantees they stop and chase.
In any case, even if he gets one or two casts off every time you have to reroot you should be able to heal/regen it back while you're sitting around waiting on DoTs/root to fade.
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