If you are a long time Plasma user, you probably remember the times when most GTK applications in KDE Plasma prior to 5.18 were practically unusable due to the lacking support for _GTK_FRAME_EXTENTS. In this blog post, I would like to get a bit technical and walk you through some changes that happened during the 5.18 time frame that made it possible to support _GTK_FRAME_EXTENTS in KWin. I also would like to explain why after so many years of resistance, we had finally added support for client-side decorations. So, buckle up your seat belt!
What is _GTK_FRAME_EXTENTS, anyway?
A window can be viewed as a thing that has some contents and a frame around it with a close button and so on. The “server-side decoration” term is used to refer to a window frame drawn by the window manager (KWin). If the application draws the window frame by itself, then we refer to that window frame as a “client-side decoration.”
A cool thing about client-side decorations is that they are very eye candy, but there is also a lot of drawbacks about them, for example if the application hangs, the user won’t be able to close the window by clicking the close button in the window frame. But the biggest issue with client-side decorations is that the window manager has to know the extents of the client-side drop shadow otherwise things such as window snapping and so on won’t work as desired.
_GTK_FRAME_EXTENTS is a proprietary GTK extension that describes the extents of the client-side decoration on each side (left, right, top, and bottom). From the word “proprietary” you have probably already guessed that _GTK_FRAME_EXTENTS is a thing that is not in any spec. We can’t afford implementing a proprietary extension simply because we don’t know whether re-designing KWin will pay off in the end. What if GTK ditches _GTK_FRAME_EXTENTS for something else and our hard work will be for nothing? There were some suggestions to standardize _GTK_FRAME_EXTENTS in the NETWM spec, but it didn’t go well.
So, what did change our minds?
It might come as a surprise, but the reason why we decided to add CSD support after so many years of being reluctant was Wayland. In order to fully implement the xdg-shell protocol (the de-facto protocol for creating desktop-style surfaces), we must support client-side decorated windows. Prior to that, we didn’t have any reason that could possibly somehow justify the changes that we would have to make in code that was battle-tested for many years.
With Wayland, we know what changes have to be done in order to add support for client-side decorated clients. Surprisingly, the geometry abstractions that we chose specifically for client-side decorated Wayland clients turned out to be also pretty good for client-side decorated X11 clients, so we decided to add support for _GTK_FRAME_EXTENTS since it didn’t require any huge changes.
It still means that we will be screwed if GTK switches to something completely different on X11, though. But let’s hope that it won’t happen.
CSD and KDE Plasma
“But Vlad,” you may say. “Does this mean that KDE is going to switch CSD?” No, as far as I know, nothing has changed, we still use server-side decorations. Support for client-side decorations was added because it’s something that we need on Wayland and to make GTK applications usable on X11.
Frame, buffer, and client geometry
Warning: This is a brain dump. Please skip to the next section if you’re not interested in technical stuff.
For an old school window manager such as KWin, client-side decorated windows are troublesome mainly because due to the long history all rendering related code had been written with the assumption that the window frame wraps the window contents. If an application draws the window frame on its own, that’s not the case.
In 5.18, we tackled that problem by introducing two new geometries to separate window management from rendering – the frame geometry and the buffer geometry.
The frame geometry describes a rectangle that bounds the window frame. It doesn’t matter whether the window is client-side decorated or server-side decorated. This kind of geometry is used practically for everything, for example window snapping, resizing, etc. KWin scripts see and operate on this geometry.
The buffer geometry is used primarily during rendering, for example to build window quads, etc.
In 5.20, we introduced yet another new geometry, which existed prior to that in an implicit form – the client geometry. The client geometry indicates where the window contents inside the window frame  is on the screen. We use this geometry primarily for configuring windows.
It can be a bit challenging to deal with three different geometries at the same time, but there is not that much we can do about it, unfortunately. Each geometry has its own specific domain where the other geometries are inappropriate to use.
CSD is a rather controversial subject in the KDE community, but it’s here and it’s not going anywhere, anytime soon.
 On X11, the client geometry actually corresponds to the geometry of the client window.