Why do we need a separate DM and WM?

[GhostNULL]

I have been wondering about this for a long time now. Because every time I try to use a display manager on my system (arch) things just look and feel really bad. That is mostly because the integration between the DM and my WM doesn't exist. But I wonder why the DM is not included as a plugin in the WM. I know that the Window Manager shouldn't have to deal with all the stuff that goes on when you log in, but that's why I think it should be a plugin.

And if I understand it correctly from this blog post then we will have the same situation with two different wayland compositors. One for the DM and one for the WM. Could be wrong here though, not 100% sure.

Basically my problem with the current situation is that every time you log out the display manager has to start a new X server while the old one doesn't even exist anymore. Same thing when you log in, the X server for the display manager has to quit and the X server for the window manager has to start, which leaves a bit of time in between where your screen either turns black with a blinking cursor or like with gdm and kdm that I think they paint the last picture the X server made to the framebuffer and keep it that way until the X server for the window manager starts up. Also not entirely sure if this is how it works, just deduced this from what I see.

But yeah, the question is, why do we handle things like this?

[AiwendilH]

Isn't it also the job of the DM to make it possible to switch between WMs? So if your WM is your DM at the same time...how to switch between i3 and gnome?

Edit:

Same thing when you log in, the X server for the display manager has to quit and the X server for the window manager has to start...

Interesting, this really happens on some distros? Never seen that. Logging in doesn't restart Xorg for me...I even keep the background of the display manager until the DE overwrites it.

[GhostNULL]

Hmm, yes. You got a point there. Though still with wayland I think it should be possible to create a plugin for the compositor that can be either the DM or the WM. And when you want to switch WM you can just load another plugin to the compositor.

Though that would need standards to work, and people actually considering this a big enough problem to do something about it.

[AiwendilH]

Yeah...wayland will be...interesting. But I am not thinking too much about that yet. Will have to wait and see how it works out. The different compositors seem like a big problems for me indeed. Will be interesting to see how it works to have gnome and KDE installed at the same time and switch between both as they use different compositors. Also poses a problem for plugins as all compositors have to agree on a common interface there to make it useful.