Issue Summary

After Upgrading Confluence from 9.0.x to 9.1.x, Default Theme "Match System" shows overlapping of the Page content while scrolling the Page down.

This is reproducible on Data Center: (yes)

Steps to Reproduce

1. Set up Confluence 8.5 and create content with a large Page size. (create multiple Users for testing)
2. Go to Admin > Layouts > Main Layout > Create Custom > Save
3. Upgrade Confluence from 8.5 to 9.1.1
4. Confluence 9.1.1 has the default Theme Set to "Match System"
5. Login with "admin" user, When you scroll down the Pages, it overlaps the Page content with the title bar. Issue Replicated locally.
6. But If I switch back from Dark/Light Theme to Match System, the issue is gone. I am not able to replicate it anymore with the User- "admin".
7. Then I switch the user to "test1" it is still replicable.
8. Then I restarted the Confluence - It is not reproducible for the admin (because I have switched the theme, but it is replicable with user "test1".
9. The issue is not reproducible with plain vanilla 9.1.1, only reproducible with the upgraded instance.

Customer Impact: Themes are user-specific, and Confluence 9.1.x has the default Theme is "System Match" and all the users are reporting the issue.

Expected Results

Page content should not overlap with "Header-banner" or Title Bar.

Actual Results

Page Content are getting overlapped while scrolling down.

Workaround

Backup custom existing layout then click Reset Default. Reapply individual changes made from custom into new layout.

Switching between the Theme to Light or Dark will fix the issue for the User.