Codex Desktop: support continuous vertical scrolling in PDF sidebar viewer

Feature request

Codex Desktop's right-side PDF viewer currently presents PDFs one page at a time. To move between pages, the user has to click the previous/next page controls.

For reading papers, reports, and other multi-page documents, this is much less convenient than the standard PDF interaction used by browsers and macOS Preview.

Desired behavior

Please add a continuous vertical scrolling mode to the PDF sidebar viewer:

• Mouse-wheel scrolling should move naturally through the current page and continue onto the next/previous page.
• Trackpad two-finger scrolling should work the same way.
• Pages should be stacked vertically in a continuous document view rather than requiring explicit page-button navigation.
• The existing previous/next page controls can remain available for quick jumps.
• Ideally, preserve the current scroll/page position when switching away from the PDF and back.

A single-page mode could remain available as an option, but continuous scrolling would be a much better default for document-reading workflows.

Why this matters

Codex is increasingly useful for workflows involving papers, generated reports, specifications, and other PDFs. When reviewing a long PDF alongside a Codex conversation, repeatedly moving the pointer to click a page-navigation icon interrupts reading flow. Continuous trackpad/mouse scrolling would make the sidebar substantially more usable for document-heavy work.

Expected interaction

1. Open a multi-page PDF in the right sidebar.
2. Scroll downward with the mouse wheel or trackpad.
3. When reaching the bottom of page N, scrolling continues directly into page N+1.
4. Scroll upward to move back across page boundaries.

This should behave similarly to the continuous-scroll modes in browser PDF viewers and macOS Preview.

Related issues

• #22095 — file viewer scroll-position / keyboard paging behavior
• #26931 — inconsistent PDF viewer behavior depending on how the PDF is opened

Those appear adjacent but do not specifically request continuous vertical cross-page scrolling for PDFs.

Start by locating the Codex Desktop PDF sidebar viewer and reviewing related issues #22095 and #26931 for existing scrolling and position behavior. Reproduce the current page-at-a-time navigation with a multi-page PDF, then verify that mouse-wheel and trackpad scrolling cross page boundaries while the existing controls remain usable.