Resolve "Unitless number in CSS `calc()` causes the computation to fail"

What does this MR do and why?

Closes #349654 (closed)

Unitless values in calc() can only be used in multiplication/division or with other unitless numbers, like:

• width: calc( 25px / 2 );
• z-index: calc( 1 * 3 );

Computations like calc( 10px - 5 ); result in an invalid value, and the entire calc being discarded.

Problem

Diff file headers in Merge Requests don't stick to the top of the screen as you scroll.

Cause

A computation that was switched from implicit in Sass to explicitly calc() includes a unitless number.

Fix

This MR adds px to the 11 value;

MR acceptance checklist

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