Copy as Rich Text from Markdown Preview

[aahlijia]

Body

Purpose: When copying a selection from the Markdown Preview, write both plain text and HTML to the system clipboard. Paste targets that understand rich text (Notion, Google Docs, Apple Notes, email clients) will receive formatted output; plain-text targets (Terminal, Notepad, vim) are unaffected.

Background:
This is standard behavior in browsers and document editors. Copying from a rendered view and pasting into a rich-text target should preserve the visual formatting. Currently, Zed strips all formatting on copy, which forces users to manually reformat headings, bold text, and lists after pasting.

Prior art:

• VS Code's Markdown preview copies rich text when targeting rich-text applications.
• Every major browser copies HTML to the clipboard when copying from a rendered page.
• This feature has been discussed in the community: [link to any relevant Zed issues/discussions if they exist].

Implementation approach:

• Add ClipboardEntry::Html(String) to GPUI's clipboard abstraction (one new enum variant + one helper method on ClipboardItem).
• Generate HTML from the selected markdown source using pulldown_cmark, which is already a workspace dependency.
• Write the HTML entry to the system clipboard on macOS (public.html), Linux X11 (text/html atom), Linux Wayland (text/html MIME type), and Windows (HTML Format with CF_HTML header).
• Scoped to the Markdown Preview only, the editor, terminal, and AI panel are not affected.

Platform support: macOS, Linux (X11 + Wayland), Windows, all three implemented.

What else this affects:

• No new actions, keybindings, or settings.
• No UI changes, behavior change only on copy.
• No theme or accessibility impact.
• Works identically in all display modes.

[jeorb]

This is a feature I used a lot in VSCode and it would be great to have in Zed.

Many things support Markdown these days but a lot of "enterprise" apps and documentation systems don't so being able to copy and paste rich text from the Markdown preview saves a conversation step when using Zed to write docs for those systems.

PR: #56452