Description:
I was on mate, and i tried to install cinnamon with

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pacman -Syu cinnamon
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but, when i launch cinamon after connection with lightdm, i have this popup :

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You are currently running in fallback mode.
If you suspect the source of the crash is a local extentsion, applet, or desklet, you can disable downloaded add-on using the checkbox below.

Do you want to restart cinnamon
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Hello,
If i restart cinnamon by check the box or not, I have this message again.
if i click on "no" cinnamon start, but it very look like mate... same panel, cinnamon menu do not appear. There is no windows decoration, and keyboard shortcuts do not work, so i can't close windows openned.

I uninstalled xf86-video-intel like suggested in wiki, but no changes...

Finaly i did that after looking for a soluce on the web :

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pacman -D --assdeps $(pacman -Qqe) && pacman -D --assexplicit base linux linux-firmware && pacman -Rsn $(pacman -Qdtq)
rm -rf /home/USER/* /home/USER/.*
pacman -Syu base linux linux-firmware
pacman -S lightdm cinnamon
´´´

And now, the only change, is that when i click on "no" i only have a black screen...

Does anybody can help me to fix that ?