Diagnosing and Resolving Package Conflicts Between Ubuntu LTS Base and KDE Neon Repositories

Recognizable condition

After running apt upgrade or apt full-upgrade the command aborts with messages such as “held broken packages”, “conflicting packages”, or “The following packages have been kept back”. You may also notice that Plasma Shell crashes after a partial upgrade or that plasmashell --version shows a version older than the latest Neon release.

Cause and diagnostic table

Symptom	Likely cause	Diagnostic command
apt upgrade aborts with conflicts	Neon repository provides newer plasma*, kwin*, qt* packages whose dependencies are not satisfied by the Ubuntu LTS base	apt -s upgrade && apt policy
Plasma shell crashes after partial upgrade	Mixed Qt/KDE library versions from Neon and Ubuntu repos	ldd /usr/bin/plasmashell | grep -E 'qt|kde'
The following packages have been kept back	Packages are explicitly held or version-locked	apt-mark showhold

Ordered checks

1. Refresh package index and simulate a full upgrade: sudo apt update && sudo apt -s full-upgrade. Look for any “conflict” or “break” lines in the output.
2. Check repository priority for a key package (e.g., plasma-desktop): apt policy plasma-desktop. The candidate version should come from the Neon archive (http://archive.neon.kde.org) and have a higher priority than the Ubuntu version.
3. List any held packages: apt-mark showhold. If the list is not empty, note the packages.
4. Examine version mismatches for core components: apt policy plasma-desktop kwin qtbase5-dev. Compare the Neon candidate with the installed version.
5. Verify that no third-party PPAs are enabled that could introduce incompatible Qt versions: ls /etc/apt/sources.list.d/ and inspect each file.

Fixes tied to findings

Version-mismatch conflicts

If the simulation shows conflicts because Neon packages are newer but dependencies are unsatisfied, allow the package manager to resolve dependencies by performing a full upgrade:

sudo apt full-upgrade

This command may install additional packages or remove obsolete ones. Review the summary before confirming.

Held or pinned packages

When apt-mark showhold returns packages that are blocking the upgrade, remove the hold and retry:

sudo apt-mark unhold 

Replace with each held package name, then run sudo apt full-upgrade again.

Third-party PPA interference

If a PPA in /etc/apt/sources.list.d/ provides competing Qt or KDE libraries, disable it:

sudo mv /etc/apt/sources.list.d/.list /etc/apt/sources.list.d/.list.disabled

Then update the package index:

sudo apt update

Afterward repeat the full-upgrade step.

Broken package state

If the package manager reports broken packages after an interrupted upgrade, attempt to fix the state:

sudo apt --fix-broken install

This will try to complete any pending installations or removals.

Escalation criteria

• Conflicts persist after running sudo apt full-upgrade, removing all PPAs, and clearing held packages.
• Critical system packages such as systemd, libc6, or glibc appear in conflict messages.
• Plasma fails to start after a reboot; you reach a login screen but the desktop does not load.
• Errors indicate repository metadata corruption (e.g., “Hash Sum mismatch”, “404 Not found” for Neon archives).

Verification and limitations

After applying a fix, run:

• sudo apt update && sudo apt full-upgrade -y and confirm that apt-mark showhold returns no output.
• Reboot the machine and check that Plasma starts: plasmashell --version should match the version advertised by the Neon repository (visible via apt policy plasma-desktop).
• Verify repository priority: apt policy plasma-desktop shows the Neon archive as the candidate with a higher pin priority than the Ubuntu version.

Limitations: This guide assumes you are using the KDE Neon User Edition repositories. Mixing the Developer Edition repositories can lead to incompatible Qt versions and is not covered here. Always back up /etc/apt/sources.list.d/ before moving or disabling files, as an incorrect change can prevent apt from locating any repositories.