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Commit 96496a86 authored by Martin Flöser's avatar Martin Flöser

Add a new markdown README

The old readme was targetted at application developers having
conceptional questions about window management. While we all know that
this is very often needed, I doubt that any application developer is
looking at KWin's README to figure out why their window doesn't get

The new README is focussing more on potential new contributors by
describing what KWin is and how to reach our team. E.g as a nice small
document readable on git project hosting sides.

The information in the readme is mostly taken from

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision:
parent d595f36e
- A collection of various documents and links related to KWin is at .
- The mailing list for KWin is (
- If you want to develop KWin, see file HACKING.
- File CONFIGURATION includes some details on configuring KWin.
- Below is some info for application developers about application interaction
with the window manager, but it'd need some cleanup.
This README is meant as an explanation of various window manager related
mechanisms that application developers need to be aware of. As some of these
concepts may be difficult to understand for people not having the required
background knowledge (since sometimes it's difficult even for people who
do have the knowledge), the mechanisms are first briefly explained, and
then an example of fixing the various problems is given.
For comments, questions, suggestions and whatever use the
mailing list.
Table of contents:
- Window relations
- how to make the window manager know which windows belong together
- Focus stealing prevention
- how to solve cases where focus stealing prevention doesn't work
properly automatically
Window relations:
(For now, this explanation of window relations is mainly meant for
focus stealing prevention. To be extended later.)
All windows created by an application should be organized in a tree
with the root being the application's main window. Note that this is about
toplevel windows, not widgets inside the windows. For example, if you
have KWrite running, with a torn-off toolbar (i.e. a standalone toolbar),
a file save dialog open, and the file save dialog showing a dialog
for creating a directory, the window hiearchy should look like this:
KWrite mainwindow
/ \
/ \
file save dialog torn-off toolbar
create directory dialog
Each subwindow (i.e. all except for the KWrite mainwindow) points to its
main window (which in turn may have another main window, as in the case
of the file save dialog). When the window manager knows these relations,
it can better arrange the windows (keeping subwindows above their
main windows, preventing activation of a main window of a modal dialog,
and similar). Failing to provide this information to the window manager
may have various results, for example having dialogs positioned below
the main window,
The window property used by subwindows to point to their mainwindows is
called WM_TRANSIENT_FOR. It can be seen by running
'xprop | grep WM_TRANSIENT_FOR' and clicking on a window. If the property
is not present, the window does not (claim to) have any mainwindow.
If the property is present, it's value is the window id of its main window;
window id of any window can be found out by running 'xwininfo'. A window
having WM_TRANSIENT_FOR poiting to another window is said to be transient
for that window.
In some cases, the WM_TRANSIENT_FOR property may not point to any other
existing window, having value of 0, or pointing to the screen number
('xwininfo -root'). These special values mean that the window is transient
for all other windows in its window group. This should be used only
in rare cases, everytime a specific main window is known, WM_TRANSIENT_FOR
should be pointing to it instead of using one of these special values.
(The explanation why is beyond the scope of this document - just accept it
as a fact.)
With Qt, the WM_TRANSIENT_FOR property is set by Qt automatically, based
on the toplevel widget's parent. If the toplevel widget is of a normal
type (i.e. not a dialog, toolbar, etc.), Qt doesn't set WM_TRANSIENT_FOR
on it. For special widgets, such as dialogs, WM_TRANSIENT_FOR is set
to point to the widget's parent, if it has a specific parent, otherwise
WM_TRANSIENT_FOR points to the root window.
As already said above, WM_TRANSIENT_FOR poiting to the root window should
be usually avoided, so everytime the widget's main widget is known, the widget
should get it passed as a parent in its constructor.
(TODO KDialog etc. classes should not have a default argument for the parent
argument, and comments like 'just pass 0 as the parent' should go.)
Focus stealing prevention:
Since KDE3.2 KWin has a feature called focus stealing prevention. As the name
suggests, it prevents unexpected changes of focus. With older versions of KWin,
if any application opened a new dialog, it became active, and
if the application's main window was on another virtual desktop, also
the virtual desktop was changed. This was annoying, and also sometimes led
to dialogs mistakenly being closed because they received keyboard input that
was meant for the previously active window.
The basic principle of focus stealing prevention is that the window with most
recent user activity wins. Any window of an application will become active
when being shown only if this application was the most recently used one.
KWin itself, and some of the related kdecore classes should take care
of the common cases, so usually there's no need for any special handling
in applications. Qt/KDE applications, that is. Applications using other
toolkits should in most cases work fine too. If they don't support
the window property _NET_WM_USER_TIME, the window manager may fail to detect
the user timestamp properly, resulting either in other windows becoming active
while the user works with this application, or this application may sometimes
steal focus (this second case should be very rare though).
There are also cases where KDE applications needs special handling. The two
most common cases are when windows relations are not setup properly to make
KWin realize that they belong to the same application, and when the user
activity is not represented by manipulating with the application windows
Also note that focus stealing prevention implemented in the window manager
can only help with focus stealing between different applications.
If an application itself suddenly pops up a dialog, KWin cannot do anything about
it, and its the application's job to handle this case.
Window relations:
The common case here is when a dialog is shown for an application, but this
dialog is not provided by the application itself, but by some other process.
For example, dialogs with warnings about accepted cookies are provided
by KCookieJar, instead of being shown by Konqueror. In the normal case,
from KWin's point of view the cookie dialog would be an attempt of another
application to show a dialog, and KWin wouldn't allow activation of this
The solution is to tell the window manager about the relation between
the Konqueror main window and the cookie dialog, by making the dialog
point to the mainwindow. Note that this is not special to focus stealing
prevention, subwindows such as dialogs, toolbars and similar should always
point to their mainwindow. See the section on window relations for full
The WM_TRANSIENT_FOR property that's set on dialogs to point to their
mainwindow should in the cookie dialog case point to the Konqueror window
for which it has been shown. This is solved in kcookiejar by including
the window id in the DCOP call. When the cookie dialog is shown, its
WM_TRANSIENT_FOR property is manually set using the XSetTransientForHint()
call (see kdelibs/kioslave/http/kcookiejar/kcookiewin.cpp). The arguments
to XSetTransientForHint() call are the X display (i.e. QX11Info::display()),
the window id on which the WM_TRANSIENT_FOR property is to be set
(i.e. use QWidget::winId()), and the window id of the mainwindow.
Simple short HOWTO:
To put it simply: Let's say you have a daemon application that has
DCOP call "showDialog( QString text )", and when this is called, it shows
a dialog with the given text. This won't work properly with focus stealing
prevention. The DCOP call should be changed to
"showDialog( QString text, long id )". The caller should pass something like
myMainWindow->winId() as the second argument. In the daemon, before
the dialog is shown, a call to XSetTransientHint() should be added:
XSetTransientForHint( QX11Info::display(), dialog->winId(), id_of_mainwindow );
That's it.
Non-standard user activity:
The most common case in KDE will be DCOP calls. For example, KDesktop's DCOP
call "KDesktopIface popupExecuteCommand". Executing this DCOP call e.g.
from Konsole as 'dcop kdesktop KDesktopIface popupExecuteCommand" will lead
to showing the minicli, but the last user activity timestamp gained from events
sent by X server will be older than user activity timestamp of Konsole, and
would normally result in minicli not being active. Therefore, before showing
the minicli, kdesktop needs to call KApplication::updateUserTimestamp().
However, this shouldn't be done with all DCOP calls. If a DCOP call is not
a result of direct user action, calling KApplication::updateUserTimestamp()
would lead to focus stealing. For example, let's assume for a moment
that KMail would use this DCOP call in case it detects the modem is not
connected, allowing to you to start KPPP or whatever tool you use. If KMail
would be configured to check mail every 10 minutes, this would lead to minicli
possibly suddenly showing up at every check. Basically, doing the above change
to kdesktop's minicli means that the popupExecuteCommand() DCOP call is only
for user scripting. (TODO write about focus transferring?)
Simply said, KApplication::updateUserTimestamp() should be called only
as a result of user action. Unfortunately, I'm not aware of any universal
way how to handle this, so every case will have to be considered separately.
# KWin
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.
# Contacting KWin development team
* mailing list: [](
* IRC: #kwin on freenode
# Support
## Application Developer
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.
# Reporting bugs
Please use [KDE's bugtracker]( and report for [product KWin](
# Developing on KWin
Please refer to [hacking documentation]( for how to build and start KWin. Further information about KWin's test suite can be found in [](
## 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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