[BUG] Desktop app: SendUserFile images render as blank cards in split view (macOS)

Environment

• Claude Code 2.1.220, running inside the Claude Desktop app
• macOS 15.7.4 (Darwin 24.6.0), Apple Silicon
• Model: claude-opus-5

Bug

Images delivered with SendUserFile render as empty cards when the desktop window is in split view. The card frame and filename area appear, but the image content is blank. Un-splitting the window makes the same images display normally — the files themselves are fine.

In the session where this happened, four PNGs (1200x630 OGP mockups, ~40-90 KB each) were sent in one SendUserFile call with display: "render". The user reported "png見れないです" (can't see the PNGs), and later identified the trigger themselves: "claude デスクトップで、window 分割するとみれなくなる" (in Claude Desktop, they become invisible when the window is split).

Steps to reproduce

1. Open Claude Code in the Claude Desktop app.
2. Put the window into split view.
3. Have the agent call SendUserFile with one or more .png paths and display: "render".
4. The cards appear, but the image content inside them is blank.
5. Leave split view — the same images now render.

Expected

Images render inside their cards regardless of split view.

Note

This is not the same as #78709 (Windows 11, thumbnail missing on the file card, image still opens on click, no display parameter). Here the trigger is specifically split view on macOS, and the card renders but its content area is empty — which points at a layout/measurement problem in the split-view pane rather than thumbnail generation.

Reported on behalf of a user via Claude Code. The agent could not observe the rendering directly; the symptom description above is the user's, confirmed by follow-up question.