Enforce a max width on markdown (md) previews

[whalesalad]

What are you proposing?

On large displays when only one file is open, viewing a markdown preview spans the full width of the display. It's difficult to read content when a line of text might span the entire width of the display.

My proposal would be one of:

1. The floating "full screen" view of a file can be resized by dragging the left or right bar inward. The box will size proportionally, so if you drag the left side closer to the center by 100px, the right side would also move closer to the center by 100px. This would give users a laser fast way to a) fullscreen a md doc and then b) drag the edges in a bit to suit their width perference
2. Or, if a single markdown preview file is open cap the width of the content to X pixels and make this a configurable setting.

Yes, I could just resize the overall window to shrink but I regularly have N files open in a columnar view + sidebar and would prefer to have an experience that is constrained to an open MD preview file, either in a standard window/tab or in the full screen toggle. Toggling into full-screen to get a "zen mode" experience for a preview would work.

Why does this matter?

This will improve the user experience with reading markdown documents inside of Zed.

Are there any examples or context?

Some example (complaints) on other platforms:

• Provide a setting for markdown preview width microsoft/vscode#193761
• https://gitlab.com/gitlab-org/gitlab/-/work_items/21797

And some screenshots in a follow-up comment.

Possible approach

My possible approaches are listed above in the proposal. I don't have any literal/tactical technical guidelines.

[whalesalad]

vs

[mkykode]

+1 to this — wanted to add a few suggestions that build on the original proposal but aren't already covered:

1. Reuse the existing preferred_line_length setting. Rather than introducing a new pixel-based max-width setting, it would be nice if the Markdown preview respected the preferred_line_length value that Zed already uses for the editor. That keeps line-length behavior consistent between the source file and its preview, and avoids users having to configure two separate values.

2. Measure the constraint in characters/ems, not pixels. A character-based width scales correctly with font size and zoom level, whereas a hardcoded pixel cap can feel too narrow or too wide depending on the user's font settings.

3. Add horizontal padding with the content centered. When the preview pane is wider than the configured line length, the content should stay centered (or consistently left-aligned with a gutter) so the layout is predictable regardless of pane width. This is what makes the reading experience feel consistent across files.

4. Consistency across all Markdown files, not just fullscreen. The original proposal focuses on fullscreen / single-file views, but the same readability issue applies in any wide pane. Ideally the setting would apply everywhere the preview is rendered, so switching between files or layouts doesn't change how the content wraps.

5. Prior art — Obsidian's "Readable line length". Obsidian handles this really well: it constrains rendered content to a comfortable width and adds padding on either side regardless of window/pane size, giving a consistent book-like reading experience. Worth looking at as a reference for both the default behavior and the toggle UX.

Example of what the setting could look like:

{
  "markdown_preview": {
    "use_preferred_line_length": true
  }
}

I'll attach a couple of screenshots showing the current full-width behavior vs. the desired constrained/padded layout.
Obsidian

Zed

[robinojw]

Hey @whalesalad I think I might have pushed up a PR for something similar: #54003 (comment)