Btw, I'm sorry if this issue should have been assigned to a different package... Plasma vs Kwin vs "?"

If that would be the case, please edit the "Product" and "Component" above. Thanks!

Excuse my ignorance, but where do you configure "single window mode"?

There is no activation for this mode. Kate, for example, can show on Desktop 1 one window and each new file will be opened in the new tab. However if you open file while on Desktop 2 it will open new windows for Kate on that desktop.

Ideally it'd be nice if we could have a feature to open file in programs that have support for tabs in the same instance but in different tab even if they are on a different desktop(unless checked otherwise in the settings of Task Manager or Icon only Manager, etc).

And i guess in theory it is possible atleast for KDE programs and maybe some others if they'll give an option to do it(a list of such programms will be pretty small to begin with so why not?).

Thanks, sunweb, for helping clarifying the point :)

But why is this a Plasma bug? Kate decides to open a new window or a new tab, doesn't it?

I think it's something that is not exclusively dependent of Kate, as it behaves differently if called from the same virtual desktop it is already open (kate opens a tab), or from a different one (kate opens a new window; note that I refer to Kate because you referred to it, but this is the case also for browsers).

So this may depend on the application and kwin or plasmashell in some sort of way... to at least make kate know about a same vs a diff virtual desktop.

What I proposed as the ideal way to tackle this, would be to adjust behavior in concordance with plasma's taskbar configuration. In the taskbar one can configure which applications are visible, whether all of them, or whether those of the same activity/desktop or screen. It would be great to have kate and browsers to open additional windows in sync with the taskbar configuration.

I hope I was a bit clearer this time...

Tried to set Special Application Settings... -> Size & Position -> Settings Activity -> Apply Initially -> Activity #2 - in Kate, Chromium and Firefox apps - then switching to Activity #1 and opening text and html files in above-mentioned apps.

My observations:
Kate app - switching to Activity #2 happens and Kate receives focus - for first opened file if before Kate wasn't started - and for subsequent files opening in new tabs in single Kate window - if Kate was started before

Firefox and Chromium apps - switching to Activity #2 isn't happening and these browsers didn't receive focus for first open file - if browsers weren't started before. For subsequent files and if they are already started in Activity #2 - switching to Activity #2 happens, browsers receive focus and file is opening in new tab in each browser's single window.

Confirmed; see also Bug 408919.

Not sure if this can be fixed generically, or if it needs fixes from each individual app.

Indeed, it needs support in each app. Bug 408919 tracks doing it for Dolphin (which is now fixed on X11, and only waiting for a fix on Wayland). Please submit a new bug report for each other affected app. Thanks!