markdown: Fix heading sizes and spacing in preview

[saberoueslati]

Context

Heading sizes in the markdown preview were nearly identical to body text, making # H1, ## H2, and ### H3 visually indistinguishable. The root cause was in MarkdownStyle::themed(): it set a heading_level_styles override with font sizes of rems(1.05–1.15), which silently replaced the correct sizes applied by apply_heading_style (via GPUI's text_3xl/text_2xl/text_xl utilities rems(1.875/1.5/1.25)). Removing that override restores the intended hierarchy. A mt_4() top margin was also added so consecutive headings have visual breathing room.

Closes #54358

Video of manual test below :

Screencast.from.2026-04-21.02-22-12.webm

How to Review

crates/markdown/src/markdown.rs Three changes:

• (1) removed the heading_level_styles block from MarkdownStyle::themed() that was overriding heading font sizes with nearly-body-text values;
• (2) added mt_4() to the heading div in push_markdown_heading for better vertical spacing;
• (3) added test_heading_font_sizes_are_distinct which renders H1–H3 and a paragraph then asserts that line heights strictly decrease from H1 down to body text.

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed heading sizes in the markdown preview to correctly reflect the H1–H6 size hierarchy, matching standard markdown renderers

[saberoueslati]

@danilo-leal, looking for feedback, @miguelraz suggested parity with VSCode might be needed in the issue #54358

The difference I found so far is the line under Heading 1 and 2, and the font is bolder in VSCode, as you see below. Should I add those?

Screenshots for reference below.

Zed screenshot

VScode screenshot

[ChristopherBiscardi]

parity with VSCode might be needed

We don't need to exactly match everything vscode or another software does, especially with respect to bold or horizontal rules. Let's keep this scoped to the heading size differences so that headings like h1 vs h2 can be differentiated in the preview.

[ChristopherBiscardi]

This looks better than it was to me. We can always make more improvements in the future. There's also another PR open to make some of these customizable that might land.

[robinojw]

Hi @ChristopherBiscardi noticed you just merged this, I've also updated the markdown rendering, any chance you could help find someone to review? #54003

[ChristopherBiscardi]

@robinojw yes, I'm already looking at it. That's the PR I meant when I mentioned one above

[yokhoe]

Finally on this one. Thought I was taking crazy pills. Thanks for the quick turn around