Rendered Markdown files can't be tabs in the tabbed editor pane: switching to Rendered discards the other tabs, and grouping is ignored for .md files

Pre-submit Checks

• I have searched Warp bugs and there are no duplicates
• I have searched Warp known issues page and my issue is not there

Describe the bug

Warp's tabbed editor pane ("Group files into single editor pane") has no room for a rendered Markdown file. As soon as a file is shown in the Markdown Viewer it lives in its own single-file pane, which breaks the tabbed editor in two ways:

1. Switching a tab to Rendered throws away the other tabs. With several files grouped in one editor pane, open a .md as a tab and click Rendered. The whole tabbed pane is replaced by a one-file viewer pane, and the other tabs are no longer visible.
2. Grouping is ignored for Markdown files. With "Open Markdown files in Warp's Markdown Viewer by default" on, every .md clicked in the Project Explorer opens a new split (50/50, then 33/33/33 …), even though the grouping setting is on and its description says "any files opened in the same tab will be automatically grouped into a single editor pane".

I read the source on master (98551490f) to confirm this is structural rather than a settings problem:

• The tabbed pane is CodeView (app/src/code/view.rs), and each TabData can only hold a LocalCodeEditorView (the raw text editor).
• A tab whose file renders in the notebook viewer gets the Rendered/Raw control, but choosing Rendered emits PaneEvent::ReplaceWithFilePane, which swaps the entire CodePane for a standalone FilePane (replace_file_pane_with_code_pane / replace_code_pane_with_file_pane in app/src/pane_group/mod.rs).
• WorkspaceView::open_file_notebook (app/src/workspace/view.rs) never consults prefer_tabbed_editor_view; it only dedupes an identical path and otherwise calls add_pane_with_direction(Direction::Right, …), so each rendered Markdown file becomes another split.

To reproduce

1. Settings → Code → Editor and Code Review: layout = Split Pane, Group files into single editor pane = on, Open Markdown files in Warp's Markdown Viewer by default = on.
2. Open the Project Explorer in a repo with a few .md files and a couple of code files.
3. Click two code files → they group into one tabbed editor pane (correct).
4. Click a .md file → it opens a new split pane instead of a tab.
5. Click a second .md → a third pane.
6. Alternatively: with the .md open in the tabbed pane in Raw mode, click Rendered → the tab strip and other tabs disappear.

Expected behavior

A rendered Markdown file should be a tab inside the tabbed editor pane, exactly like a code file:

• Opening a .md from the Project Explorer with grouping on adds a tab to the existing editor pane (a preview tab on single click, pinned on double click — the same rules code tabs already follow). No new split.
• Rendered / Raw toggles the content of that tab in place. Sibling tabs stay.
• Users who want several Markdown files open flip between tabs; users who want "just show me this one" get it through the existing preview-tab behaviour. No new setting needed.
• EditorLayout::NewTab and the standalone viewer pane keep working as they do today.

Sketch of the change (happy to open the spec/PR): make the tab content polymorphic (TabContent::Editor(LocalCodeEditorView) | TabContent::Rendered(FileNotebookView)), route open_file_notebook through the grouped CodePane when grouping is on, make the Rendered/Raw toggle swap the tab content instead of the pane, no-op editor-only concerns (save prompts, LSP status, diff accept/reject) for rendered tabs, and add a rendered flag to CodePaneTabSnapshot for restore.

Screenshots, videos, and logs

Screenshots of the split-on-every-click behaviour are on #8965 (my earlier report about the window/pane sizing side of this).

Operating system (OS)

macOS

Operating system and version

macOS 27.0

Current Warp version

v0.2026.09.09.08.26.stable_02

Regression

No, this has been the behaviour since the Markdown Viewer shipped

Additional context

Related: #8965 (window should extend by a preferred viewer width instead of splitting the terminal — this issue is the prerequisite for that), #13396 (file tabs in a single native editor pane, which the grouping setting already covers for code files; this issue is specifically about rendered Markdown), #11918.

Does this block you from using Warp daily?

No, but it keeps me from using the built-in Markdown Viewer at all.

Is this an issue only in Warp?

Yes