According to the RRS definitions: Proper Course A course a boat would choose in order to sail the course and finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.
In this case, you were certainly sailing your proper course. The J/105's proper course, however, would likely be different from yours. In order to go faster on a reach they might head 10º higher than you. What happens then?
The rule that pertains in this situation is rule 17:
ON THE SAME TACK; PROPER COURSE
If a boat clear astern becomes overlapped within two of her hull lengths to leeward of a boat on the same tack, she shall not sail above her proper course while they remain on the same tack andoverlapped within that distance, unless in doing so she promptly sails astern of the other boat. This rule does not apply if the overlap begins while the windward boat is required by rule 13 to keep clear.
Rule 13 involves boats that are tacking, which does not seem to apply here. So... the interpretation of the rule depends on how the overlap was established. Were they overlapped while well to leeward and worked their way to windward, closer and closer? Or did the J/105 come up essentially from behind you, within two boatlengths? If the former, they could head you up head to wind if they wanted. If the second, they have to maintain THEIR proper course - "in the absence of the other boats referred to in th rule using the term". It's as if your boat wasn't there. Your proper course has nothing to do with this situation. They are leeward boat and you, as windward, must keep clear. If their proper course is 10º higher than yours, they can head 10º higher than you, and you have to keep clear. They can't head higher than their proper course, (or they'd have to promptly sail astern of you), but the fact that they're going past you is proof that they're trying to finish as soon as possible and are on their proper course, even if YOU think they might be overstanding.
This happened to us in a race, where we were the leeward boat, reaching towards a mark in our J/36 and passing a Sabre 362 within two boatlengths. (Don't know how they got ahead of us. Maybe a staggered/pursuit start?) Like you, they didn't want to alter course above THEIR proper course, and protested us. I knew the rule, ignored the yelling, and passed him. They withdrew their protest when the protest committee explained to them that they were in the wrong. You are lucky the J/105 skipper wasn't up on his rules.