[BUG]: App hangs when calling WinMM MIDI output APIs on a disconnected device

[m-komo]

Windows Version

Windows 11 25H2 build 25220.7653 (Insider Beta)

Service Installation Method

In-box In Insider Build

SDK version, if appropriate

No response

Location

Application using WinMM MIDI 1.0 (Classic API)

Type of bug

Application not working as expected

Steps to reproduce

1. Connect any USB MIDI device.
2. Launch PocketMIDI and select the device connected in step 1 as the Output Port.
3. Confirm that MIDI messages can be sent successfully from PocketMIDI.
4. Disconnect the device.
5. Attempt to send a MIDI message from PocketMIDI.

The application becomes unresponsive (hangs).

Expected behavior

Even if the API call is made to an invalid (disconnected) MIDI port, the call should immediately return with an error code, and the application should not become unresponsive.

Additional notes

In environments where the MIDI Service is enabled, calling midiOutShortMsg or midiOutLongMsg on an invalid MIDI port results in a 5‑second timeout wait.
In previous environments (without the MIDI Service enabled), the API calls returned immediately with an error code.

Would the following fix address this issue?

#823

[dmiura]

We met same issue with Cubase 15 and ProtoZOA (UUT_USB_MIDI_ECHO_NO_PRINTF.uf2.zip) .

1. Made two MIDI tracks in Cubase project. (test.cpr.zip)

• Track #1 is for sending Note On/Off recorded messages to ProtoZOA(MIDI Output set "ProtoZOA")
• Track #2 is for for receiving MIDI loop-back from ProtoZOA(MIDI input set "ProtoZOA")

2. Confirm that MIDI messages can be successfully sent on Track #1 and received on Track #2 by playbacking Track #1.
3. Disconnect the USB cable from PC remaining playbacking Track#1

Then, Cubase becomes unresponsive (hangs).

[Psychlist1972]

@dmiura @m-komo This looks like the surprise removal issue which should be fixed in the update going out at the end of February. I messaged you in Teams about this so you can try the internal windows dll (the change is in WinMM) under NDA.