I've got 2 macs, a laptop and a 27 inch iMac. I used to play Runescape on them a while back and only after doing so on them, they each had the same problem. The mouse wouldn't click (and I tried multiple "mice") so I therefore couldn't open any windows. I reinstalled mac on the laptop and the problem was fixed, but on my iMac I did nothing and it somehow went away on its own eventually. Just out of curiosity, could this have been Runescape's fault or was it just a coincidence? Thanks.

Runescape itself didn't. The source was most likely a 2nd party program you were trying to use to play the game ( a bot or something ). Those things are full of keyloggers & hackers.

There is a possibility that it is because of the Game. Kindly Play only popular games from popular companies which you have downloaded/got from trusted sources. Do not play Trial Version Games.