The upstream README's sudo just install fails on CachyOS because rustup is user-installed and root has no default toolchain. Here's what works.
sudo pacman -S libxkbcommon libxkbcommon-x11 just rustup default stable
git clone https://codeberg.org/bhh32/cupola.git cd cupola just build-release sudo cp target/release/cupola /usr/local/bin sudo cp data/.desktop /usr/share/applications sudo cp data/.svg /usr/share/icons/hicolor/scalable/apps xdg-mime default org.codeberg.bhh32.Cupola.desktop image/png image/jpeg image/gif image/webp image/bmp image/tiff image/x-icon update-desktop-database ~/.local/share/applications 2>/dev/null sudo update-desktop-database /usr/share/applications pkill cosmic-files
Reopen COSMIC Files and images will now open in Cupola on double-click.
The justfile has install: build-release, so sudo re-invokes cargo as root. Root has no default rustup toolchain on a standard user-level rustup install. Building as user and running the three cp commands manually skips the redundant rebuild.