When May Medieval Gentlemen Display Their Breeches?

Not often, actually.

The lower orders may do so more often.  Laborers in the fields in hot weather may strip to shirt and breeches without reproach. They need to sweat to do their duty. Likewise workers in a bakery serving a hot oven, or those burning heretics, or executioners. Huntsmen on foot pushing through damp underbrush might also strip down to their shirt, shoes and breeches.

Gentlemen rarely have good cause to be so underdressed under ordinary circumstances. The most benign exception might be young gentlemen exercising themselves with sword and buckler with their hose rolled down for limberness, but this was probably done in a somewhat private setting.

Special circumstances present exceptions, of course. A gentleman might be taken prisoner and stripped to his shirt, perhaps to prevent escape, or delivered up for execution,  or required to surrender a town under his authority to a victor eager to humiliate the defender.  Or be visiting a public bathhouse.

Most of the time a gentleman had little reason to walk about with his breeches exposed. Those of us attempting to recreate a medieval gentleman should dress accordingly.

A problem arises when individuals recreating the Middle Ages combine hose attached at a single point with a short outer garment.

The inevitable result is exposed breeches. This is neither authentic, fashionably medieval, nor flattering.

And a lot of the cases of "diaper look" I saw this Pennsic could have been avoided by properly fitted single-point hose worn at the right height. The coat wasn't so very short that the breeches would have been exposed if the hose was long enough. There's no need for the back of your hose to hang lower than your gluteal fold. Except that if off-the-rack hose is sized so that it's wearable by almost everyone in that shoe size, it's shorter than it should be for most wearers. Unless you always wear a long coat, it's worth paying extra for hose that's the right length.

Also, a belt will do a better job of keeping them up than a drawstring. I favor a belt within the breeches casing when wearing long single point hose.

Further, it's important to wear the right kind of breeches. The long and baggy breeches of the Morgan Bible were very poorly suited the shorter hemlines of the late 14th century. Tighter fitted and shorter breeches were increasingly favored by the fashionable, and by the last decades of the century the Tacuinum Sanitatis shows even peasant laborers wearing short, tightly fitted breeches.

Suppliers are not technically being deceptive when they sell as 14th century breeches garments that would have been quite acceptable in 1330. You, as a buyer, need to understand how obsolete such a garment would have been for a fashionable gentleman in 1390. A century is a long time.