[Post-Install] Keyboard and mouse not working post installation of 26.04 (after upgrade from 25.04)

[notAProgrammer]

I updated from 25.04 or 25.10 to 26.04, and now my in built keyboard and mouse aren't working. They are however, working in the grub menu.In addition, the power button is correctly turning the system off.

I have two kernels - 7.0.0-27-generic - 6.17.0-40-generic. Trying to boot into the recovery mode (graphical) of the older 6.17.0-40 still leaves me with the same problem in the graphical interface. However, in recovery mode during terminal usage as root, I'm able to perform all standard I/O operations.

Any advice on what to do next?

[notAProgrammer]

[Fixed]

I sorted this out by first figuring out what the running display manager was. Now, from the root terminal area, no graphical interface was online. So I couldn't do something like

systemctl status sddm

Instead, I catted the display manager area:

cat /etc/X11/default-display-manager

After finding that the system was using sddm, I checked the config file in /etc/sddm.conf. The new isntall for some reason decided I would like to use an x11 display servers, even though my previous version (and rest of my laptop) run on Wayland.

Catting the config file:

$ cat /etc/sddm.conf
[Autologin]
Session=plasma

[General]
DisplayServer=x11

Which was a fun surprise. I swapped the DisplayServer to wayland to get it running again.

As a fast workaround you can also run:

plasmastart-wayland

In order to access your actual graphical interface (post login) and then backup your files or something.

Good luck!

[claydoh]

Odd. that setting *should* be in /etc/sddm.conf.d/10-wayland.conf. There are more settings in that file that may be relevant to the login screen.
Having sddm run in either x11 or wayland should not matter here, of course in theory. In any case, it is running a standalone instance of either until the user logs in.

the "old" /etc/sddm.conf is usually blank, but using it is fine. You may want to look in /etc/sddm.conf.d/ , maybe there are conflicting settings that may be the real cause?

My 26.04 sddm.conf is blank, it was upgraded from 24.04 (where sddm was runing in an x11 instance). I do seem to have some older files in /etc/sddm.conf.d/, and some setting options duplicated, though some of these are not actually set in one (empty options), while are defined in another.

In a fresh-ish vm, my sddm.conf only has autologin options I set, and in /etc/sddm.conf.d I have 10-wayland.conf and 20-kubuntu-conf.


10-wayland.conf

[General]
DisplayServer=wayland
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=la yer-shell

[Wayland]
CompositorCommand=kwin_wayland --drm --no-lockscreen --no-global-shortcuts --locale1​

20-kubuntu-conf

[Autologin]
Relogin=false
Session=plasma
User=

[General]
HaltCommand=
RebootCommand=

[Theme]
Current=kubuntu
CursorSize=30
CursorTheme=breeze_cursors
Font=Noto Sans,10,-1,0,400,0,0,0,0,0,0,0,0,0,0,1

[Users]
MaximumUid=60000
MinimumUid=1000

The other files I have in there on my bare metal install do have some duplicated settings, from using the "apply Plasma settings" option. I assume that these files are read in order, also assume the ones beginning with numbers are loaded *after* those without, and thus overriding them?

Which explains why the cursor size I use on the desktop (42) does not get applied, as this is set in the kde_settings.conf file, while the 20-kubuntu-conf settings take over.


so, tl;dr the ordering of how the various config files are loaded seem and the presence of past configs may be conspiring to muck things up lol.

[buttermilk]

I had the same problem with SDDM after upgrading from 25.10 to 26.04. No keyboard or mouse input on the login greeter, and after about 1 minute, the login greeter foreground (widgets, dialog, and the background blur layer) would disappear, leaving only the background and mouse cursor. I'm running a Framework laptop with an intel chipset with integrated intel UHD graphics and a desktop-grade NVIDIA card attached via a TB4 eGPU. I initially upgraded to 25.10 for the better eGPU support in Wayland (I couldn't get Wayland running smoothly in 24.04). However, in 25.10, I did tweak it some and actually loaded SDDM in X11, but Plasma in Wayland because SDDM with X11 worked better than SDDM with Wayland for whatever reason.

After the 26.04 upgrade, I spent hours messing around reading journalctl, resetting my graphics card stuff, even tried to get SDDM to work at all without the eGPU, only on the laptop with only the intel graphics (prime-select intel and other tweaks), and it still wouldn't work. I spent hours back and forth with Claude AI, sending logs and running tests and couldn't get anywhere.

Long story short, I gave up on SDDM and just switched to GNOME display manager, and it worked fine.

So, for anyone else who might encounter this problem, ask yourself how attached you are to SDDM. I'll probably check out Plasma Login Manager once it looks stable enough on Debian so I wasn't really attached to SDDM personally.

Steps in case anyone needs help installing a new Display Manager:

sudo apt install gdm3

After install, this should detect that you have multiple display managers and open a dialog to select your default, pick gdm3. If it doesn't open the dialog you can trigger it with

sudo dpkg-reconfigure gdm3

Then reboot, and now GNOME is your display manager. Note that this will also install the GNOME desktop, which will be the default. Before you log in, click on your user account, then click the gear icon and instead of Ubuntu, select Plasma or whatever graphical shell you like.