Opera version: 134.0.5954.46 (Stable, 64-bit, official .deb from opera.com)
Engine: Chromium 150 (69bf1c67cb894365d151bd020bb0171fd583633a)
OS: Linux (Ubuntu 24.04, x86_64)

Description:
Players on sites using MSE with AAC-in-MP4 (e.g. music.yandex.ru) never start. The play button is pressed, but playback never begins, with no errors shown in the console.

Steps to reproduce:

1. Open any page in Opera, open dev console
2. Run: MediaSource.isTypeSupported('audio/mp4; codecs="mp4a.40.2"')
3. Result: false

For comparison:
MediaSource.isTypeSupported('audio/webm; codecs="opus"') → true (works)

What I've already checked and ruled out:

• Widevine CDM is up to date (4.10.3050.0), libwidevinecdm.so is present, same file size as the working copy bundled with Google Chrome
• Widevine itself works (verified on shaka-player-demo.appspot.com — keys are created successfully, Widevine video plays)
• opera://media-internals/ shows pipeline_state stuck at kStarting and never progressing, with no error events at all
• Issue reproduces in both regular and private windows, with extensions disabled
• Switching User-Agent to Chrome via Network conditions did not help
• On the same machine, official Google Chrome plays the same content with no issues

It looks like in this Opera Linux build, proprietary codecs are enabled for regular playback but not for Media Source Extensions specifically. Could you confirm or let me know if there are plans to fix this in an upcoming build?

Thanks in advance!