Plasma 5.8 will bring an improvement fixing a bug reported more than a decade ago. Back then Plasma did not even exist, the bug is reported against an early KDE 3 version. The addressed problem is the handling of panels on multi-screen setups.
This is if one has multiple screens and tries to put a panel between two screens – on the shared edge – the panel does not have a “strut” set and thus windows maximize below it:
[------------][------------] | ||P | | 1 ||P 2 | | ||P | [------------][------------]
In this illustrated setup the panel is “P” and windows on screen 2 ignore the panel. What might be surprising here is that this was not just a bug, but deliberate behavior. There is code making sure that the panel on the shared edge gets ignored. Now one doesn’t write code to explicit break useful features, there’s obviously a good reason for that.
And to understand that we must look at how panels and there struts work. First let’s look at Wayland. Wayland doesn’t have a concept for panels or struts by default. KWin provides the PlasmaSurface interface which Plasma can use to give a window the role “Panel” and to describe how the panel is used: whether it’s always on top, or whether windows can cover it or go below. KWin can use that to decide whether the panel should have a strut or not. Thus on Wayland KWin was able to support the setup shown above since it supports panels.
On X11, though, we have the NETWM spec which describes how to set a partial strut:
The purpose of struts is to reserve space at the borders of the desktop. This is very useful for a docking area, a taskbar or a panel, for instance. The Window Manager should take this reserved area into account when constraining window positions – maximized windows, for example, should not cover that area.
The start and end values associated with each strut allow areas to be reserved which do not span the entire width or height of the screen. Struts MUST be specified in root window coordinates, that is, they are not relative to the edges of any view port or Xinerama monitor.
Now here we see already the problem: it’s not multi screen aware. The strut is specified in root window coordinates. So in our case above we would need to set a strut for the left edge which spans the complete height. So far so good. But the width of the strut must be specified in root window coordinates which includes the complete screen 1. If Plasma would set this, we would have a problem.
In Plasma 5.7 KWin’s strut handling code got slightly reworked to perform sanity checks on the struts and to ignore struts affecting other screens. Basically KWin broke the implementation of given spec and in multi-screen setups only allows struts which make sense.
Now at least KWin could handle this situation properly, but Plasma still has the check to not set a strut on shared edges. For Plasma 5.8 we now changed the condition: if the window manager is KWin we allow such struts. For any other window manager we still go with the previous solution. We still think that we cannot just set a strut which would in the worst case exclude a complete screen. As that’s how the spec is written, we need to assume the window manager is standard compliant. For KWin we know that it is not standard compliant any more and support such struts, so Plasma can make use of it.
This change hopefully improves the multi-screen experience for our Plasma users who use KWin as a window manager.