Twenty Nineteen: Replace whitespace character for heading decoration.

[KarunyaChavan]

Trac ticket: https://core.trac.wordpress.org/ticket/65433

When a user inserts an empty heading block in the editor and applies an "Underline" or "Strikethrough" text decoration, the browser attempts to underline the pseudo-element's whitespace character. This interaction causes a stray, visible hyphen-like artifact to render inside the editor above the empty block as

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What this Changes

• This PR updates the post-section-dash mixin in sass/mixins/_mixins-master.scss to use an empty string (content: "";) instead of content: "\020".
• By removing the whitespace character, the pseudo-element no longer contains an actual text node.
• Since the element has explicit display: block, width, and height properties, the decorative dash continues to render identically on the frontend and backend.
• However, because there is no text content, the browser can no longer underline a whitespace character, completely resolving the visual bug in the editor without requiring any editor-specific CSS overrides.

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): GPT-4o
Used for: To verify optimality and efficiency of the patch.

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