301507 2025-10-27 02:22:03 -0700 REGRESSION (Safari 26): MediaRecorder emits empty Blobs for small timeslice values 2026-02-25 23:20:52 -0800 1 1 1 Unclassified WebKit Media Safari 26 All macOS 26 RESOLVED FIXED InRadar P2 Normal --- 1 register jean-yves.avenard byron jean-yves.avenard jer.noble webkit-bug-importer youennf oldest_to_newest 2154404 0 register 2025-10-27 02:22:03 -0700 When using MediaRecorder.start(timeslice) with small timeslice values (e.g. ≤ 100 ms), Safari 26 on macOS 26 (Tahoe) and iOS 26 consistently fires dataavailable events containing empty Blobs (blob.size === 0). Only after ~1 second has passed will such an event contain a blob with the combined data from the full second. Larger timeslice values (≥ 1000 ms) work as expected, producing valid data. This regression did not occur on Safari on macOS 15/iOS 18, where small timeslice intervals produced usable chunks. Reproduction: 1. Open the minimal repro sandbox: https://codesandbox.io/p/sandbox/youthful-benz-jsxdzx?file=%2Findex.html%3A323%2C1 2. Grant microphone access. 3. Set Timeslice = 50 ms and press Start. 4. Observe console/log output — multiple dataavailable events fire with blob.size === 0. 5. Repeat with Timeslice = 1000 ms — valid non-empty Blobs are produced. Expected: Each dataavailable event should deliver a valid Blob containing encoded media, even for small timeslice values. Actual: dataavailable events fire but contain empty Blob objects (size 0) until the recorder is stopped or the slice value is increased. Notes: requestData() suffers the same problem and cannot be used as a workaround (can be tried in CodeSandbox as well). This issue showed while working on an implementation of a live visualization of voice input. 2154449 1 webkit-bug-importer 2025-10-27 08:51:43 -0700 <rdar://problem/163489898> 2175685 2 jean-yves.avenard 2026-01-27 22:41:40 -0800 Per spec ``` If timeslice is not undefined, then once a minimum of timeslice milliseconds of data have been collected, or some minimum time slice imposed by the UA, whichever is greater, start gathering data into a new Blob blob, and queue a task, using the DOM manipulation task source, that fires a blob event named dataavailable at recorder with blob. ``` [1] Emitting zero blob is I believe the correct behaviour. WebKit uses a 100ms minimum time slice. dataavailable has to be fired every 100ms, but webkit will not generate segment lesser than 1s long since bug 282320. [1] https://www.w3.org/TR/mediastream-recording/#mediarecorder-methods 2175689 3 jean-yves.avenard 2026-01-27 22:44:20 -0800 the limitation to 1s segment is that all segments generated by the MediaRecorder will start with a keyframe. 2182619 4 jean-yves.avenard 2026-02-18 00:05:53 -0800 Pull request: https://github.com/WebKit/WebKit/pull/58910 2185109 5 ews-feeder 2026-02-25 23:20:49 -0800 Committed 308266@main (a18b47068781): <https://commits.webkit.org/308266@main> Reviewed commits have been landed. Closing PR #58910 and removing active labels.