[FEATURE] Desktop app: make the rendered Markdown preview editable in place #98103

Preflight Checklist

• I have searched existing requests and this feature hasn't been requested yet
• This is a single feature request (not multiple features)

Problem Statement

In the desktop app (Code tab), opening a .md file gives a rendered preview that is read-only. To change anything I have to switch to source mode or right-click and "Open in" an external editor.

For a workspace that is Markdown for majority of files (notes, plans, specs, briefs) this is a massive friction point. I would start reading in the preview, and when I have to edit something I need to switch, find the paragraph I was looking at (because switching mode will always bring you back to the top), change it there and then try to not get annoyed by reading in source version. Or I open it in another app right away. Whichever I go for: the flow of reading a doc Claude just wrote and updating it on the spot is impossible.

Other tools in this space (Open AI's desktop app including Codex, Nimbalyst, Notion etc.) treat an editable MD preview view as the baseline. I really enjoy a lot about the Claude desktop app, but still consider switching back to another option like CLI in Nimbalyst to avoid this problem.

macOS, desktop app 2.9939.4.

Proposed Solution

Let the rendered Markdown preview accept text edits directly (WYSIWYG-style, the way the OpenAI desktop app and Codex do), writing back to the file on save with the same Save button / shortcut the source view has today.

Minimum useful version: plain text editing inside paragraphs, list items, headings and table cells, with the Markdown syntax kept on write-back. Rich formatting controls are not needed - MD formatting would work just fine.

Alternative Solutions

• Source mode with Save. Works, but it is a raw text box, and it is what you switch away from the moment you want to read.
• "Open in" external editor. Works, but every edit leaves the app, and the request to add custom apps to that menu (Desktop app: allow custom applications in the file-link "Open in" menu (e.g. Typora) #82251) exists mostly because of this gap.
• Split view (source left, preview right, scroll-synced). Better than today, still two panes for one document.

Priority

High

Feature Category

Desktop app (Code tab) file viewer

Use Case Example

Claude drafts a project brief as brief.md. I open it, read it in the preview, and spot a wrong date in the third paragraph and a bullet that should go. Today: toggle to source, find the paragraph again (position is not kept, see #89155), edit, save, toggle back to read on. Wanted: click into the paragraph, fix the date, delete the bullet, save, keep reading.

Additional Context

Related open issues, none of which is this request:

• Feature request: extend plan-mode's inline-comment review UI to arbitrary markdown files #82643 asks for plan-mode's inline comment UI on any Markdown file (annotate, not edit).
• [FEATURE] File viewer: keep line position when toggling between source and preview modes #89155 asks to keep line position when toggling source and preview. An editable preview removes the toggle for most edits, so that request largely disappears.
• Desktop app: allow custom applications in the file-link "Open in" menu (e.g. Typora) #82251 asks for custom apps in the "Open in" menu, which is the current workaround for the same pain.