After SDDM login: Black screen, only mouse

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May be this is the Mesa issue described in the latest update thread:

Look for “Graphical issues on older hardware (Black screen, Flickering) due to mesa 25.2.x update” in the thread and press the small triangle at the left.

This is the workaround:

It seems on older hardware the mesa 25.2.x update created some regressions which may result in black screens or flickering. Some older Nvidia cards with nouveau driver seem to be affected.

You can try to use the 25.1.x series of Mesa via this repo. Simply place it on top of [extra] in your /etc/pacman.conf file and use sudo pacman -Syuu to downgrade.

[mesa-251]
SigLevel = PackageRequired
Server = https://download.manjaro.org/pkgs/mesa

If you have no graphical session available you can access the bootloader by pressing ESC key during boot and select the bootloader entry. Press CTRL+E for edit and search for the words quiet and splash. Delete those and add 3 to the line. Then execute CTRL+x to start your session. You will land in a text console TTY. Login with your user.

Edit the /etc/pacman.conf with nano: sudo nano /etc/pacman.conf. Perform the downgrade and reboot.