According to the Hanafi madhhab, what matters is whether the ʿawrah itself is exposed, not whether an inner garment becomes visible.

If during rukūʿ or sujūd your trousers or thawb shift and the waistband or upper portion of your boxer shorts becomes visible, but the skin between the navel and the knees remains completely covered, then your ʿawrah remains covered and the prayer is valid.

The fact that the boxers themselves are visible does not invalidate the prayer. The requirement is that the ʿawrah be covered by an opaque covering that conceals the skin.

As for tight clothing, the Hanafi jurists distinguish between:

• Clothing that is opaque and covers the skin but outlines the shape of the body.
• Clothing that is so thin or transparent that the color of the skin can be seen through it.
• The latter is not sufficient. However, the former, although disliked if excessively revealing of body contours, still fulfills the basic requirement of covering the ʿawrah.

Therefore, if your boxer shorts:

• are not transparent,
• prevent the skin from being seen,
• and continue covering the area of the ʿawrah,
• then the exposure of the boxers themselves does not affect the validity of the prayer.

In the Hanafi school, a prayer is only endangered when the actual ʿawrah becomes exposed. Even then, the jurists discuss detailed conditions regarding the amount exposed and the duration of the exposure before invalidity is established.

Based on the situation you described, where only the boxers become visible while the skin remains covered, your ʿawrah is still considered covered and your ṣalāh is valid.

You should not concern yourself with repeating prayers because of this.