I've been having this issue for a couple days now where I'm tumbling the viewport and it will suddenly either flip completely upside down or rotate off kilter until I use space+G to reset it. It's happening so frequently now that is making it difficult to work. The only keys i'm using to navigate are Alt and Middle / Left click. See below :

https://imgur.com/8NbPGYw [imgur.com]

Answered a couple days ago here [www.sidefx.com].

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Excellent. Thank you! Thought that was on by default.