Sushi Browser’s mainstay definitely is its tab splitting feature. This involves dividing the browser window’s frame between two or more tabs.
There are other power-user-oriented browsers which do this, such as Vivaldi and Tungsten, but Sushi arguably has the most dynamic approach.
Vivaldi and Tungsten can orient tabs vertically and horizontally also. Horizontal orientation probably is the less common selection of the two, but it does have its use when comparing content that need the screen width, like PDF documents.
The downside of Vivaldi’s tab splitting is that the subsequent panels cannot be resized. If for example you have two tabs split vertically, one contains a PDF which does overfill the panel, and the other has a site with responsive web design, you would have to update the zoom level of the PDF or scroll horizontally as you view it.
Tungsten does allow you to resize panels, but because Tungsten is a one-window browser, you only can do this with its only window. It has tab grouping as does Vivaldi to compensate for this, but this means that if you have panels, then you’d have to make a tab group just to have the option to view a webpage by using the browser’s entire window. I’d personally rather have multiple windows than tab groups like Sushi does.
In a way, Sushi actually does have tab grouping through its sidepanel feature. You can add tabs to the left, right, or bottom sidebar and then collapse the sidebars when you want to view the main content in the entire browser window. It’s nice that Sushi keeps the tabs as-is even when the sidebars are collapsed. The tabs do not need to be reloaded in any sort of way at least not because of the browser itself.
Personally, the multi-panel feature is the one that has become mandatory for any browser I use long-term. I use it for leisure as well as for productivity. It makes multitasking with a single- or dual-monitor configuration more manageable. Vivaldi and Tungsten (and Otter Browser, as well) have their own multi-panel integrations, which I’m glad they offer to users. While I do prefer Sushi’s functionality, it’s fortunate that there are multiple browsers with this feature.