Nic, options don't trade like stocks. Several things have happened over those 4 days. First, August options have closed; current month is now September. Second, much of the pressure is moving forward to September and October (which just opened) which takes pressure off of December. Much of the news/rumors suggest that a deal will be known by September expiration or possibly into October - this takes the pressure off December and hence the drop in contract premiums for that month.
Options are very tricky to trade. If you bought December $105's because you're only playing for the deal, then you've got no worries and a little longer time frame for the deal to get done serves you better. If you were trying to trade in and out, then you've just experienced the problem with trading options as news comes out and new months open up for trading.
Don't worry about the current value if you believe that Interdigital is close to selling the company.
I wish you luck with your strategy.
IMO
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