"Uh, would've been funnier if they skipped the last panel" - random commenter

Or 2 panels on top of 1 "widescreen" panel could work to

I can think of a few possibilities

1. Some sites like Instagram were/are specifically restricted to square images only (I think that's changed now but I don't use instagram)
2. When images are very wide or very tall, apps will often fill to a differently aspected thumbnail, cutting off important parts of the comic like dialogue. My Lemmy client (Connect) does this, for example. If your base image is square, then the thumbnail will also always show the whole image.
3. A square image is a compromise which is fine on both mobile and desktop and is a middle ground everyone can tolerate.

I think some do a vertical scroll, but that's usually lengthier and used for things with narrative. On Lemmy for instance I see Owl House do a long scroll, but it's more of a light novel than a comic strip.

A three panel vertical.... Actually isn't Cyanide and Happiness doing that, in particular on Lemmy feeds?

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The joke's in the fourth panel.