[BUG] macOS: clicking a chat-sent video thumbnail opens it twice — in-app lightbox + separate browser tab

What's Wrong?

Opening (clicking) a video that was sent in the chat as a file attachment renders two simultaneous copies on screen:

1. An in-app expanded/lightbox video pane, full-bleed, taking up part of the window.
2. A separate browser tab/window that the app spawns itself, pointing at the same local file path — with its own tab bar, URL bar (showing the local /Users/.../CloudStorage/Dropbox-.../filename.mp4 path), and forward/back/reload controls.

Both copies play the same video at the same time, side by side, effectively splitting the screen.

This was reproduced with zero browser/preview-pane tool calls in that turn — the file was delivered purely via a standard "send file to user" action, with no explicit request to open a browser or preview pane. Clicking the resulting chat thumbnail was enough to trigger the duplicate render, which confirms the duplication is caused by the thumbnail's own click/open handler in the client, not by any tool-driven navigation.

Steps to Reproduce

1. On macOS, have Claude send a local video file (.mp4) to the user as a chat attachment (e.g. a Dropbox-synced file).
2. In the chat, click the resulting video thumbnail/attachment card to open/preview it.
3. Observe: the video opens in an in-app expanded video pane and in a brand-new browser-style tab within the app, both showing the same file at the same local path, at the same time.

What Should Happen?

Clicking a sent video's thumbnail should open exactly one preview surface for that file, not two.

Environment

• Claude desktop app version: 1.30096.5 (/Applications/Claude.app)
• macOS 26.6.1 (Build 25G76)
• File source: local file synced via Dropbox (CloudStorage/Dropbox-...), .mp4, ~50MB

Possibly related

• #81386 — Windows-only bug about the Browser/preview pane itself painting two clipped copies of a page, tied to Windows display-scaling / devicePixelRatio mismatches. That repro is different (it's about the Browser tool's preview pane rendering, not about opening a sent-file attachment), and doesn't involve Windows scaling factors here — but the symptom of "same content rendered twice at once" is similar enough that the two may share a root cause in how the app manages duplicate render surfaces.