Combining Long Vertical Line Overlay

Put it directly after the symbol it modifies.

For relations such as =, <, >, ∈, or ⊂, it forms a negated relation: the base sign tells you what is being negated. Keep the two characters adjacent, with no space or line break; this is the vertical negation bar pattern.

When the ordinary single-character form exists, use it: ≠ is the standard not-equal sign, while =⃒ is a base sign followed by this overlay.

Why does the overlay move or disappear?

The font and renderer set its exact height and alignment.

This is a nonspacing mark: its stroke sits over the preceding symbol and has no independent width. Fonts can change the stroke's length or slant, and rendering can misplace it or fail to show it. A display glitch does not change the intended relation.

Where did the long vertical line overlay come from?

It belongs to a standards tradition for mathematical negation.

The standards trail reaches a 1991 update to ISO 9573-13, which defined mathematical symbols for SGML, a system for tagging document structure. That work used overlays to write negated relations compactly, including forms without a single precomposed character. The mark belongs to an evolving standards lineage, not a one-off invention.