[BUG] Desktop app artifact side pane: mouse-wheel scroll dead on first load of a republished artifact version (matches #74954 symptom)

Summary

In the Claude Code desktop app on macOS, the artifact side pane stops responding to mouse-wheel scrolling after an artifact is republished (new version) and the pane loads that version for the first time. The main conversation pane scrolls normally; only the artifact pane is dead to the wheel. The artifact's own page was ruled out as the cause (details below).

This looks like the same host-side iframe height/scroll measurement race previously reported in #74954 (Safari, first load, fixed by switching the version picker). That issue was closed as "not about Claude Code" — this report is the desktop-app side-pane surface of the same symptom, per that issue's closing guidance to open a new issue if still relevant.

Environment

• Claude Code 2.1.206, macOS 26.5.2 (Darwin 25.5.0), desktop app side pane (artifact opened next to the conversation)
• Artifact: private HTML artifact, single-column document ~3300 px tall (taller than the pane viewport)

Steps to reproduce

1. Publish an HTML artifact taller than the viewport and open it in the desktop app's side pane.
2. Republish the same artifact (same URL, new version) via the Artifact tool.
3. The side pane loads the new version. Try to scroll it with the mouse wheel.
4. Nothing scrolls in the artifact pane. The conversation pane on the left scrolls fine at the same time.

What was ruled out on the artifact side

The exact same HTML was tested in Chrome under three hosting architectures, with real wheel events:

1. Standalone page — wheel scrolls normally.
2. Fixed-position pane with an inner-scrolling iframe — wheel scrolls normally.
3. Content-height iframe (scrolling="no") inside an outer scroll container — wheel chaining to the outer scroller works normally.

The page contains no overflow: hidden, no fixed heights, no position: fixed, no wheel/touch event handlers, and no overscroll-behavior overrides.

Workaround

Per #74954: using the artifact's version picker to switch versions (even reselecting the same version) restores scrolling. Closing and reopening the pane also helps.

Expected

Wheel scrolling works in the artifact side pane on first load of a newly published version, without needing to toggle the version picker.