Issue header isn't fixed when scrolling

Summary

When viewing an issue from the project view, scrolling the issue down won't maintain its header fixed.
Alternatively, when opening an issue from top panel Issues>Issue key and scrolling, the summary header will also not be fixed on this instance.

Steps to Reproduce

1. Access a project from Projects menu (top bar)
2. Click Issues on the left (project navigation)
3. Select one of the listed issues
4. Scroll the issue down

or

1. Click on top bar Issues>Issuekey to open an issue
2. Once the issue opens, scroll down

or

1. Navigate to Issue Navigator and search for issues
2. Right click and open an issue in a new tab
3. Scroll the issue down

Expected Results

While scrolling the issue down the header of the issue, containing the summary and buttons, should stay fixed.

Actual Results

The header scrolls up.

Notes

See attached screenshot for element inspection
Fixed header:

Not fixed:

Workaround

Access the results from the Issue Navigator (i.e View the issue navigator results in 'List view') and then click on the issue key. DO NOT do a right click>open in a new window.