Partial Flash sometimes equals Endless Flash

When using the new partial-flash feature, I've experienced this symptom twice on a Win 10 system.

- Click Flash
- Dialog gets stuck like this
- Editor area (of app) is ghosted
- micro:bit's yellow serial activity LED blinks constantly (random rate, not fixed cadence)
- No recovery except to disconnect USB cable

Retested on Chromebook: no problems.

Retested on Win 10: can't reproduce.

Console didn't seem to have any helpful messages. Is there a way for me to provide more debugging information should this happen again?

No file, test, or entry point is named. Start by exercising the partial-flash flow through Flash on Windows 10 and inspect the editor and serial-activity behavior; done means identifying a reproducible cause or collecting actionable diagnostics for the stuck dialog.

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