I like the side pane where you can even have a second web page(Web panel) in it while surfing in the main pane. If I wasn't on a mid sized laptop and had a decent size monitor, I'd probably have X always open in the side pane.
I might have to make the swap to Vivaldi anyway. Web Panel would be handy in the early mornings when the only lights I use is a few red LEDs to light up the coffee maker area. The LEDs are on a smart relay that connects to the Ethernet on my router and I can turn it on/off via the browser at a local numeric IP address.
I used a PeppermintOS app called Ice to create a SSB(site-specific browser) for my LED lights. It's a browser stripped of it's menu, address bar and other features to make it more like an app. It uses less resources that way. Ice is for Debian/Ubuntu users. Ice also creates a menu item for your application/programs menu and you can either choose an icon for that or it will use the favicon or it's default icon. By default, it will stick the menu item/icon under the Internet section of the menu but you can choose a different section.
My LED switch web page as an SSB and I did use Vivaldi for it but you can also use Firefox or Brave/Chromium/Chrome. 
If I had a big monitor, I would probably have X and FR as SSBs and then have a full browser for general surfing. I have SSBs for RokuTV, PlutoTV & TubiTV websites but rarely use them. I do use one for TuckerCarlson.com and run it on a different virtual desktop and just listen as I surf for the most part.
Ice on github - https://github.com/peppermintos/ice
Ice on Launchpad where a deb install file is available - https://launchpad.net/~peppermintos/+archive/ubuntu/ice-dev/+packages
SSBs on wikipedia - https://en.wikipedia.org/wiki/Site-specific_browser
Since my Kubuntu menu is fully editable, I could create SSBs manually by adding a menu item and copying what Ice created. 
Chromium/Chrome version is the same of course. 
As is Brave 
I have Firefox installed as a Snap/snapd application and Ice doesn't see it so I can't create a FF SSB so I don't know what the command is but as you can see above, the command to start any Chromium based browser as a SSB is the same and the same commands can be used in a terminal and any url could be swapped in. --app= is the trigger.
Kind of bugging me about firefox. Seems to only be available as a Snap or Flatpak these days, neither of which Ice will see. Might have to fire up an old laptop and put Ice on it and make a FF SSB to see the command. Firefox removed the ssb feature a few years ago so maybe it simply won't work now. I know I've done it before but it's been a while.