Learn Swedish: Resten av inlägget handlade om uppbrottet. - The rest of the post was about the breakup.

It comes from the particle verb bryta upp (to break up). To make it a noun, the particle upp shifts to the front, and the verb stem bryta changes to its noun form brott (a break). This gives us ett uppbrott (a breakup), which becomes uppbrottet in the definite form.

Is inlägget (the post) formed using the same rule?

Exactly. It comes from the particle verb lägga in (to put in, to submit). Moving the particle in to the front of the stem lägg creates the noun ett inlägg. While it historically means a contribution or insert, it is now the standard Swedish word for a blog or social media post.

Why doesn't handla om (to be about) form a noun like omhandel?

Handla om is a prepositional verb, not a particle verb. In spoken Swedish, the stress falls on the verb (handla), not the preposition (om). Because om isn't a stressed particle here, it can't shift to the front to form a noun in the way that upp and in do in this sentence.

Why do we need av in resten av inlägget when Swedish usually drops "of" in phrases like "a cup of coffee"?