The goal of KDE when it was first started, was to create a consistent desktop. Where all the look, feel and interaction from one app was the same as the next app.
I’ve seen lots of mockups or discussions which use Plasma components inside desktop applications, the result not only looks inconsistent but prevents a usability problem.
(FIXME mockup showing what I’m talking about)
Not only do our applications risk looking un-seemless with ourselves (FIXME choose a real word), but we destroy all the work that has gone into trying making our applications seamless on Gnome, OS X and Windows. Using traditional widgets we adopt the look and feel of the native toolkit, a form layout will even be aligned differently on each system to match the natvie approach.
Plasma components were designed and built for the workspace domain. Panels, applets and so on. When merged inside the desktop, we end up with behaviour that stops being consistent from one to the next. Layouts are different, keyboard accelerators and shortcuts don’t work, views scroll different and items which provide the same functionality are visually disimilar, requiring the user to re-learn how to use a widget.
It’s wrong to be using desktop widgets in the workspace domain, and equally it’s wrong to be using workspace widgets inside applications. As a general rule;
“if it has window decorations, user interface elements should look like ‘application’ widgets”. – Aurelien Gateau
This isn’t about the technology, there’s scope to use QML inside an application if you are trying to make something visually different and new, but the end result needs to fit in with everything we currently have.