Widget and windeco themes for Plasma Next

[Thomas Pfeiffer]

Hi everyone,
I admittedly injected the topic of widget and windeco themes for Plasma 
Next into a thread about cursor themes, so now I'm starting this thread 
to properly separate the two topics. Bear with me, it's gonna be rather 
long...

=== What's the current situation? ===

We now have a working Plasma theme (Breeze) as well as a working cursor 
theme (Bridge) which can be shipped with Plasma 2016.06. What we don't 
have yet, though, are widget themes for both QtWidgets and 
QtQuickControls as well as a window decoration theme.

The QtQuickControls style is almost complete now thanks to Andrew Lake 
and our awesome design forum community.

The goal is of course to have a QtWidgets theme which matches the 
QtQuickControls style. Since the VDG lacks C++ coding skills, there is 
currently a QtCurve preset in the works by forum user garthecho that 
tries to mimick the style as closely as possible.

For window decorations, we have a set of quite impressive Aurorae themes 
by garthecho [1] and Andrew is about to kickstart work on a QML-based theme.

=== Where do we go from here? ===

If we ship what we have now, we'll have Breeze and Bridge together with 
Oxygen widget and windeco theme.
While this has the advantage that Oxygen is "tried and true", it has the 
following drawbacks:
- The Oxygen QtQuickControls style in its current condition (at least 
from any Plasma Next screenshots and videos I've seen) looks unpolished 
as hell. Labels in buttons aren't aligned correctly and there are other 
problems as well. If we ship Plasma Next like this, everything looks 
beautiful... until someone opens a GUI which was made with 
QML/QtQuickControls (which is all of Plasma's own GUIs iirc). As soon as 
people do, what they get looks like a crappy version of Plasma Current 
(aka "KDE 4").
I don't think we want this. So if we want to at least be visually on par 
with current Plasma, additional work would have to go into the Oxygen 
QtQuickControls style (and according to David, a bug in Qt would have to 
be fixed)
- If we get the QtQuickControls theme to look exactly like Oxygen 
QtWidgets, Plasma Next will look at least as good as Plasma Current 
(presumably slightly better for various reasons I don't need to repeat 
here), but it doesn't really look "new".
- The "flatness" of Breeze doesn't really fit the "3D-ness" of Oxygen.

So what options do we have?
According to Martin Gräßlin, wie should not ship an Aurorae theme as 
default window decoration due to performance reasons.
What about a QML-based one? Would that be better, or does it absolutely 
have to be C++?

Would a QtCurve preset instead of a C++ widget theme work at least as an 
interim solution, or would that cause too many problems?
Would it be okay to ship the new QtQuickControls style for QML-based 
GUIs, but Oxygen for QtWidgets-based ones? what about the window 
decoration, then? Oxygen and Breeze don't really mix well...

So, you guys are the technical experts, what should we do?

Cheers,
Thomas