Based on the Kevin's design this is an extra idea that I wanted to share, with a squarer version (light) and a more rounded one (dark)

Note that this mockup appears to, in its bottom-left, provide an example of what invent.kde.org/teams/vdg/issues/-/work_items/29 would look like, implemented.

A big update is that it seems we'll have a major technical roadblock solved by the time 6.7 lands: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/6280

On the exporting the current setup as a look and feel:
Comment on a purely technical side (ignore the UI of that completely): the feature is partly already there as pure implementation in the lookandfeelexplorer tool from plasma-sdk, code in part there (creation of the package and export of other settings) and in part in plasmashell itself, as the dumpCurrentLayoutJS() DBus method.

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Dolphin still doesn't look anything like this and even that cyan border is still there. While I understand the need to redesign a major app, all the other QT apps look just as bad or worse. A lot of this is down to the theme engine.

This deserves a blog post with screenshots!

These mockups have now all been implemented :) Regarding the tabbar, I ended up with a vertical design as it worked the best with some calligra apps containing a lot of tools.

Moved to https://invent.kde.org/teams/vdg/issues/-/issues/43, where I've summarized the rough conclusion here, and we can work on turning it into something actionable quickly.

Moved to https://invent.kde.org/teams/vdg/issues/-/issues/12

This is done now for Plasma 6!

That was merged, and this is basically done in Plasma 6 now.