Mint does this when constrained within a pot - its natural growth habit is to throw out long runners up to 18 inches under the ground and pop up a good distance away from the original clump, which is why its so often said its not a good idea to plant it in open ground.
Turn it out of its pot, cut the root ball into 3 sections, replant one in the same pot. If you want more, replant those in 2 other pots. It should recover and grow anew, although, if you live in the northern hemisphere, reduced hours of daylight and possible chilliness near the window means it won't grow as fast as it would in spring or summer.
One caveat though - I'm rather wondering what those black flecks beneath the leaves in the last photo are - are they just specks of potting compost or aphid infestation? If its the latter, that needs dealing with, you can either rub them off and kill them between your fingers, or use a neem spray. If the flecks are within the leaf and not removable, suggesting a fungal problem, take care with your watering regime - give it a good water when you repot, but don't keep it wet nor allow water to remain in the outer tray for longer than 30 minutes after watering.