[Bug]: Exec in desktop entry for generix linux seems to be buggy

Checks before filing an issue

• This issue doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
• I have checked the issue tracker and have not found an issue that matches the one I'm filing.
• This issue is not a troubleshooting question. Troubleshooting questions go here: https://forum.mattermost.com/c/trouble-shoot/16.
• This issue is not a feature request. You can request features and make product suggestions here: https://mattermost.com/suggestions/.
• This issue reproduces on the most recent stable version, or the most recent prerelease version of the Mattermost Desktop App.
• I have read the contribution guidelines.

Mattermost Desktop Version

5.6.0

Operating System

NixOS 23.05

Mattermost Server Version

No response

Steps to reproduce

1. Login using Gitlab SSO.
2. Authorize Mattermost in Gitlab
3. Allow the Browser to open in...

Expected behavior

The Browser asks to be allowed to open Mattermost. We get redirected to the Mattermost Desktop App with a logged-in user.

Observed behavior

The Browser asks us to allow it to open xdg-open (instead of Mattermost). After we allow it, the default Browser is opened with the following url in the address bar:
mattermost-dev://mattermost.example.de/login/desktop?client_token=dev-<client_token>&isDesktopDev=true&server_token=<server_token>

Log Output

I forgot to save the log outputs.

Additional Information

Two things are still wrong in the current master while writing this. I'll explain what, using two patches that fixed my local installation.

1. The binaries packaged in the released tar archive use mattermost-dev:// as prefix for the mattermost desktop app, a possible fix could be to build the mattermost-desktop app with the ELECTRON_IS_DEV environment variable set to 0 to get rid of the -dev (I also explain a workaround using another MimeType in the code explained below)

modified   package.json
@@ -59,7 +59,7 @@
     "package:mac-with-universal": "npm-run-all build-prod && electron-builder --mac --x64 --arm64 --universal --publish=never",
     "package:mas": "cross-env NODE_ENV=production IS_MAC_APP_STORE=true npm-run-all check-build-config build && electron-builder --mac mas --universal --publish=never",
     "package:mas-dev": "cross-env NODE_ENV=production IS_MAC_APP_STORE=true npm-run-all check-build-config build && electron-builder --mac mas-dev --universal --publish=never",
-    "package:linux": "npm-run-all package:linux-all package:linux-appImage",
+    "package:linux": "set ELECTRON_IS_DEV=0 && npm-run-all package:linux-all package:linux-appImage",
     "package:linux-appImage": "npm-run-all build-prod-upgrade package:linux-appImage-x64 package:linux-appImage-arm64",
     "package:linux-appImage-x64": "electron-builder --linux tar.gz appimage --x64 --publish=never",
     "package:linux-appImage-arm64": "cross-env CC=aarch64-linux-gnu-gcc CXX=aarch64-linux-gnu-g++ electron-builder --linux tar.gz appimage --arm64 --publish=never",

2. The Desktop Entry Exec Attribute does not need double quotes surrounding the mattermost-desktop path. See: https://wiki.archlinux.org/title/desktop_entries#Modify_command_line_arguments

modified   src/assets/linux/create_desktop_file.sh
@@ -9,7 +9,7 @@ cat <<EOS > Mattermost.desktop
 [Desktop Entry]
 Name=Mattermost
 Comment=Mattermost Desktop application for Linux
-Exec="${FULL_PATH}/mattermost-desktop" %U
+Exec=${FULL_PATH}/mattermost-desktop %U
 Terminal=false
 Type=Application
 # I am using `MimeType=x-scheme-handler/mattermost-dev` as a workaround. 
 # The following line is still missing in v5.6.0
 MimeType=x-scheme-handler/mattermost

Also beware of the last three lines in the code above.