Jenkins 2.264: Configure UI completely broken after upgrade

TL;DR

After upgrading Jenkins to 2.264, the Configure pages (job config, configure clouds) render completely broken: layout mangled, sections unusable.

Steps

1. After upgrading Jenkins to 2.264, the Configure pages (job config, configure clouds) render completely broken: layout mangled, sections unusable. This was the tables-to-divs UI modernization in Jenkins core 2.264, which broke configuration pages of plugins that still used the old table-based markup. The EC2 plugin and branch-api were among the culprits. The core upgrade itself is fine; the breakage comes from outdated plugins rendering config forms.
2. Update the offending plugins: for the EC2 plugin you need at least version 1.54 for it to work on Jenkins >= 2.264, and similarly update branch-api and other plugins that render configuration pages. The breakage is plugin-side (old table markup vs the new div-based layout), so upgrading Jenkins core alone does not fix it; the plugin updates do. This was tracked as JENKINS-64072 and closed with resolution fixed.
3. Take the question above and check if it matches what you're seeing. If it does, work through the verified fix step by step. Start with the cause described first, because that's what tells you the fix applies to your setup, then apply the changes in the order given. Verify by re-running whatever failed before, and expect the same behavior the thread author reported.

When to use

You are seeing this: This was the tables-to-divs UI modernization in Jenkins core 2.264, which broke configuration pages of plugins that still used the old table-based markup. Use this skill when you run into "Jenkins 2.264: Configure UI completely broken after upgrade".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: 1.54. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Someone asked: After upgrading Jenkins to 2.264, the Configure pages (job config, configure clouds) render completely broken: layout mangled, sections unusable. This was the tables-to-divs UI modernization in Jenkins core 2.264, which broke configuration pages of plugins that still used the old table-based markup. The EC2 plugin and branch-api were among the culprits. The core upgrade itself is fine; the breakage comes from outdated plugins rendering config forms. The verified answer: Update the offending plugins: for the EC2 plugin you need at least version 1.54 for it to work on Jenkins >= 2.264, and similarly update branch-api and other plugins that render configuration pages. The breakage is plugin-side (old table markup vs the new div-based layout), so upgrading Jenkins core alone does not fix it; the plugin updates do. This was tracked as JENKINS-64072 and closed with resolution fixed.

Source: https://github.com/jenkinsci/jenkins/issues/11792 Discussion and confirmation live at https://vectle.com/threads/thr_QjqD-VmH5tkqJPoJb7KNVg