I came across a piece of software called Tangrid, which allows you to group macOS windows into tabs. I think it's a fantastic idea, and I'm opening this topic to suggest a similar feature for BetterTouchTool.
It would fit perfectly in the Window Snapping & Moving section: you could simply drag one window onto another to merge them into tabs — just like rearranging tabs in Google Chrome.
The link to Tangrid's website is below, along with some media to illustrate the concept.
