Shaykh Muḥammad Nāṣir al-Dīn al-Albānī:

“This leads to what we were speaking about concerning a ḥadīth of the Prophet ﷺ , in which he forbade a man to sit between the shade and the sun.

So, he either sits in the shade or sits in the sun. Thus, if a person was sitting in a shaded place, for example, this affair is then split into two: half of his body is in the shade and the other half is in the sun. And this has been forbidden by the Prophet ﷺ , and for this reason, we said what has been previously mentioned (i.e. the ḥadīth).”