I'm very fortunate that T never acts any different, in heat or not. She is a mare and for sure has an opinion, but she is well mannered and knows no shenanigans in saddle or in hand.
I was at a team penning practice with T once, and a stallion was next to us and apparently T went into heat. I felt nothing out of the ordinary, but a friend told me T was "winking" at the stallion.
The stallions owner/rider had complete control of him and there wasn't any problem.