The uncaptured-error handler from #404 was armed in `Pipeline::new`, which
`MultiPipeline::ensure_len` runs again for every pane added — so the slot was
re-registered on every new pane. Move it to `MultiPipeline::new`, which runs
once per device; if iced ever rebuilds the device it builds a new
`MultiPipeline` too, so the fresh device stays covered — where a `Once` guard
would have left it on wgpu's panicking default.
Throttle the message. The handler slot is per-device and single, so it also
catches iced's own draw errors and is the only thing that can report them — it
has to stay loud. But a validation error normally repeats on every frame, and
an unthrottled print floods stderr at refresh rate and buries the first, most
useful message. Log the 1st, 2nd, 4th, 8th … occurrence instead: the error is
never hidden, the running count shows it is ongoing, and the output stays
finite.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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