Does Mattermost have a normal web page that you can load?
If so, you can add any page you want to the sidebar. Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 horizontal dots at the bottom of the sidebar, scroll down to "Custom site panels" and add the URL to the Mattermost page.
Another workaround is to add any page you want that has a calculator to the sidebar. Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 horizontal dots at the bottom of the sidebar, scroll down to "Custom site panels" and add the URL to the page. You could add https://www.google.com/search?q=calculator to the sidebar for example to always get the Google calculator.
@andrew84 Thank a lot for clear picture.
I do not tackle sometimes such issues as I do not use the options you wrote about. Ctrl-Tab and Ctrl-Space is not what I use. I know well what pages I have kept opened and where! Well named workspaces make wonders. 😉
e.g.
(Second part relates to second window.)
Examples for enigmatic News:
News 2 - News all (US, Russia, Mil)
News 3 - News (social Novorossiya)
News 4 - News (IT, Tech, Natures etc)
News 5 - News (mainly Zen)
etc
ssox_2432.png
BTW Jocular remark on explanations.
mpv-shot0113.png
Unfortunately, this feature does not seem to be important to Opera devs, not even enough to warrant a reply to your thread. The tab cycler used to be very good with a vertical list of tabs and full-screen previews, but now it's just terrible - tiny thumbnail previews, horizontal that only displays a handful of tabs, the mouse constantly messing things up. Because of this, I have migrated from Opera to Vivaldi.