Summary:
KWindowSystem provides a plugin interface to have platform specific
implementations. So far KWin relied on the implementation in
KWayland-integration repository.
This is something I find unsuited, for the following reasons:
* any test in KWin for functionality set through the plugin would fail
* it's not clear what's going on where
* in worst case some code could deadlock
* KWin shouldn't use KWindowSystem and only a small subset is allowed
to be used
The last point needs some further explanation. KWin internally does not
and cannot use KWindowSystem. KWindowSystem (especially KWindowInfo) is
exposing information which KWin sets. It's more than weird if KWin asks
KWindowSystem for the state of a window it set itself. On X11 it's just
slow, on Wayland it can result in roundtrips to KWin itself which is
dangerous.
But due to using Plasma components we have a few areas where we use
KWindowSystem. E.g. a Plasma::Dialog sets a window type, the slide in
direction, blur and background contrast. This we want to support and
need to support. Other API elements we do not want, like for examples
the available windows. KWin internal windows either have direct access
to KWin or a scripting interface exposed providing (limited) access -
there is just no need to have this in KWindowSystem.
To make it more clear what KWin supports as API of KWindowSystem for
internal windows this change implements a stripped down version of the
kwayland-integration plugin. The main difference is that it does not use
KWayland at all, but a QWindow internal side channel.
To support this EffectWindow provides an accessor for internalWindow and
the three already mentioned effects are adjusted to read from the
internal QWindow and it's dynamic properties.
This change is a first step for a further refactoring. I plan to split
the internal window out of ShellClient into a dedicated class. I think
there are nowadays too many special cases. If it moves out there is the
question whether we really want to use Wayland for the internal windows
or whether this is just historic ballast (after all we used to use
qwayland for that in the beginning).
As the change could introduce regressions I'm targetting 5.16.
Test Plan:
new test case for window type, manual testing using Alt+Tab
for the effects integration. Sliding popups, blur and contrast worked fine.
Reviewers: #kwin
Subscribers: kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D18228
KWin is an easy to use, but flexible, composited Window Manager for Xorg windowing systems (Wayland, X11) on Linux. Its primary usage is in conjunction with a Desktop Shell (e.g. KDE Plasma Desktop). KWin is designed to go out of the way; users should not notice that they use a window manager at all. Nevertheless KWin provides a steep learning curve for advanced features, which are available, if they do not conflict with the primary mission. KWin does not have a dedicated targeted user group, but follows the targeted user group of the Desktop Shell using KWin as it's window manager.
KWin is not...
a standalone window manager (c.f. openbox, i3) and does not provide any functionality belonging to a Desktop Shell.
a replacement for window managers designed for use with a specific Desktop Shell (e.g. GNOME Shell)
a minimalistic window manager
designed for use without compositing or for X11 network transparency, though both are possible.
If you are an application developer having questions regarding windowing systems (either X11 or Wayland) please do not hesitate to contact us. Preferable through our mailing list. Ideally subscribe to the mailing list, so that your mail doesn't get stuck in the moderation queue.
End user
Please contact the support channels of your Linux distribution for user support. The KWin development team does not provide end user support.
Please refer to hacking documentation for how to build and start KWin. Further information about KWin's test suite can be found in TESTING.md.
Guidelines for new features
A new Feature can only be added to KWin if:
it does not violate the primary missions as stated at the start of this document
it does not introduce instabilities
it is maintained, that is bugs are fixed in a timely manner (second next minor release) if it is not a corner case.
it works together with all existing features
it supports both single and multi screen (xrandr)
it adds a significant advantage
it is feature complete, that is supports at least all useful features from competitive implementations
it is not a special case for a small user group
it does not increase code complexity significantly
it does not affect KWin's license (GPLv2+)
All new added features are under probation, that is if any of the non-functional requirements as listed above do not hold true in the next two feature releases, the added feature will be removed again.
The same non functional requirements hold true for any kind of plugins (effects, scripts, etc.). It is suggested to use scripted plugins and distribute them separately.
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