I personally like that behaviour (widely used in Japanese emulators).

Slightly OT, but would it be possible to assign mouse capture to the middle mouse button? It does wonders in Neko Project, and it's much more intuitive than any key combo.

Moe: I like the idea. I find it cumbersome to have to hit ctrl+f10 or (more often) alt+tab away from DOSBox to get my mouse cursor back.

Kaminari: There probably aren't many, but some games use all 3 buttons. I do think DOSBox's keymapper should be able to assign anything to anything, whether it be DOSBox actions or virtual input device actions. Unfortunately it isn't currently quite that powerful.

we use to have the mouse like this.
but it was troublesome for games that scroll using the mouse.
(that's what I recall from it)

Sure, it's not universally usable, but there's always the possibility to press Ctrl-F10 for those games, then it works like before.

I've tested it the whole day (ahem, very serious scientific research, not that you think I played games 😉 ) and found that even touch-the-border-to-scroll games work nicely.