RUN AS-IF-FOSS Bug: fix header height calculation for GlDrawer

What does this MR do?

Via !38527 (comment 392892229), we needed a better approach to rendering the sidebar with a position from top. However, on boards this doesnt account for when we are in "Focus Mode".

This is a small iteration to make it better and to play nicely with the performance bar and other header options.

though i think a css option would be a better long term solution.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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