Window Thumbnails as scaled down window content are hardly useable. Windows of same application are hardly distinguishable from each other which eliminates all advantages from thumbnails in first place.
In addition on Wayland it is difficult to get access to live-updating window thumbnails in the desktop shell. While the compositor has access to the window content, other applications do not. This would require a custom protocol to pass the window content from the compositor to the desktop shell. A protocol would be required which would be fairly close to being an own Wayland server.
This document describes how to put the application more in control of the window thumbnail which will be rendered.
The protocol will be implemented using DBus to be windowing system agnostic. The window manager registers a DBus service "org.kde.kwin.windowmetadata1" and the object path "/org/kde/kwin/WindowMetadata1". On this interface there is a call "register" which allows an application to call it and to indicate that it supports the protocol.
The window manager is able to map windows to the DBus peer through the PID. On both X11 and Wayland the window manager has access to the PID. In case of Wayland this is only possible for the window manager, thus the DBus service must be bound on the window manager.
When a thumbnail is required the window manager informs the application for which window and in which size a thumbnail is needed. In addition it passes a filedescriptor of a pipe to the application. The application renders the thumbnail and writes the rendering result into the pipe and closes the pipe once the rendering is finished. The application can render a thumbnail of different size if this of more use. To support that a small binary protocol on the pipe is required. It needs to describe:
* size (width/height) * stride * image format * byte count * binary data
Obviously the server side needs to sanity check the values.
The protocol is not meant to have life-updating previews. There is a DBus call for the application to indicate that it has new data available, but it's the responsibility of the server to initiate the process. By that the server can control the updates to a sensible amount.
Internal implementation detail, to be defined.
An additional idea is to allow the application to specify additional actions through the DBus interface. E.g. a "reload" action in a browser. This is intended to be implemented in a future release.
The implementation will be provided through a tier1 framework. It will be windowing system agnostic and only depends on DBus. For the application it will be as simple as just rendering to a QImage (through QPainter) or using a special QtQuick import.