Problem

Opening a supported file disables the main composer without a specific explanation. At the same time, Markdown editing uses a bespoke contentEditable surface, HTML↔Markdown conversion, document selection listeners, and deprecated document.execCommand.

The Canvas edit prompt also clears optimistically and does not reliably preserve text when sendMessage returns a non-throwing failure.

User impact

The assistant becomes least available when users most want to ask about a document. Editing can suffer from caret/selection quirks, paste damage, formatting loss, unclear save state, and lost edit prompts.

Relevant code

• apps/desktop/src/ui/Canvas.tsx
• apps/desktop/src/ui/ChatView.tsx
• Composer and file-context request handling
• Coordinate with the Canvas race-safe save issue

Implementation outline

• Keep normal chat available while Canvas is open.
• Add an explicit removable current-file context chip.
• Add Ask about this file, Edit this file, and future selection-context actions.
• Make it clear whether the file is attached to the outgoing request.
• Preserve chat and Canvas edit drafts independently.
• Replace the bespoke rich editor with a maintained foundation such as Lexical, ProseMirror, or Tiptap.
• Use a stable Markdown source-of-truth and a documented conversion model.
• Add accessible toolbar state, undo/redo, paste handling, selection preservation, and keyboard commands.
• Preserve edit prompts whenever sending is rejected.
• Surface Saving/Saved/Failed/Conflict states from the race-safe persistence layer.
• If a Canvas state must be read-only, explain why and provide a repair action.

Acceptance criteria

• Opening a file does not automatically disable chat.
• Users can send with or without explicit current-file context.
• Chat and editor drafts survive switching files/panes.
• Failed sends preserve prompts and context.
• Common formatting, paste, undo/redo, and selection flows are stable.
• The editor is fully keyboard-operable and labeled.
• Save/conflict state is always visible and recoverable.
• Closing Canvas does not discard an unsent chat draft.

Required tests

• Send with/without file context while Canvas stays open.
• Failed send and preserved prompt.
• Switch files with both chat and editor drafts.
• Formatting, paste, undo/redo, selection, and Markdown round-trip fixtures.
• Keyboard and accessibility coverage.

Additional scope from the July UI audit

• Make Quick preview versus Open in Canvas an explicit user choice with consistent ownership and close behavior.
• Bring PPTX/slide preview titlebar, actions, loading, and error states into the shared Canvas/Preview contract.
• Clamp selection and formatting controls to the visible viewport.