Fix shutdown banner visibility on Plugin Manager pages

[dhanush-raja-a]

What

Fixes the shutdown banner being obscured by the sticky app bar's backdrop blur effect on the Installed, Available, and Updates pages of the Plugin Manager.

Root Cause

The sticky app bar (introduced in c42ab43) uses pseudo-elements with backdrop-filter: blur(10px) that extend upwards (negative top inset). These pseudo-elements have z-index: -1 relative to the sticky bar (which has z-index: 2), but the shutdown banner had no explicit z-index (defaulting to 0), causing it to be visually obscured.

Solution

Added position: relative and z-index: 5 to #shutdown-msg and #safe-restart-msg in src/main/scss/base/_style.scss to ensure they render above the sticky app bar's backdrop effect.

This is a minimal fix (4 lines of CSS) that preserves the header redesign while ensuring critical system messages remain visible.

Fixes #26022
Closes #26024
Fixes #25949
Closes #26234

Testing done

• Identified the exact commit that introduced the issue via git bisect (c42ab43)
• Verified the last known good commit (ee56e8f) did not have the issue
• Built Jenkins locally with mvn -am -pl war,bom -Pquick-build clean install
• Ran Jenkins locally with mvn -pl war jetty:run
• Manually tested the fix by preparing for shutdown and verifying banner visibility on all Plugin Manager pages

Screenshots (UI changes only)

Before

The shutdown banner is obscured by a gradient/blur effect on the Installed, Available, and Updates pages (as shown in issue #26022).

After

The shutdown banner is now clearly visible on all 4 Plugin Manager pages, appearing above the sticky search bar.

Proposed changelog entries

• Fix shutdown banner visibility on Plugin Manager pages when preparing for shutdown.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood. Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests. (This is a visual CSS fix that requires manual verification)
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate. (N/A - CSS only change)
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable. (N/A)
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives. (N/A - CSS only change)
• For dependency updates, there are links to external changelogs and, if possible, full differentials. (N/A)
• For new APIs and extension points, there is a link to at least one consumer. (N/A)

Desired reviewers

@MarkEWaite @timja

[timja]

Thanks! I tested and also pushed a fix for it not working on the experimental dashboard page (which had a z-index of 102).

[janfaracik]

Tested, working fine for me. Thanks!

[janfaracik]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!