[BUG] Windows desktop app: show_widget ⋯ menu → 'Download file' silently does nothing (Copy to clipboard works)

Environment

• Claude desktop app 1.21459.3.0 (x64, Microsoft Store install)
• Windows 11 Pro, build 26200

Steps to reproduce

1. Open the Claude desktop app on Windows
2. In a chat, have Claude render a widget using the visualize MCP server's show_widget tool
3. Click the ⋯ (overflow) menu in the widget's top-right corner
4. Click "Download file"

What happens
Nothing. No file is saved, no Save As dialog opens, and no error or notification appears — the click fails completely silently.

What should happen
The widget saves to the Downloads folder (or a Save As dialog opens), with visible confirmation.

Key narrowing detail
"Copy to clipboard" — the sibling item in the same ⋯ menu — works correctly (confirmed: the copied content pastes fine into the composer). So the menu, the click handling, and the widget serialization are all functional. The failure is isolated to the download/save path specifically.

Evidence gathered (via Claude Code watching the filesystem during the click)

• A zero-byte .tmp file with a GUID name is created in %TEMP% at the exact moment of the click; nothing further is ever written. Reproduced twice on 2026-07-16 (~21:26 and ~22:05 local).
• Full sweep of all four drives (creation-time filter, no exclusions) confirms no file of any type or size is written anywhere.
• Window enumeration (including invisible windows) confirms no file dialog is spawned.
• The app's Preferences file (%APPDATA%\Claude\Preferences) contains no download.* or savefile.* keys, suggesting no download has ever completed on this install — i.e., broken since install, not a regression from a recent update.
• Windows' Downloads known-folder registration is correct ({374DE290-...} → C:\Users\<user>\Downloads), so this is not a folder-redirection problem on the machine.

Impact
Users get no file and no feedback, so they assume the file saved and go looking for it — the silent failure costs more time than a visible error would. Workaround: use "Copy to clipboard" from the same menu instead.