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Details

Type: Bug
Resolution: Fixed
Priority: P2: Important
Fix Version/s: 6.8.2, 6.9.0 Beta2, 6.10.0 FF, 6.10
Affects Version/s: 5.15.18, 6.8.1
Component/s: QML: Declarative and Javascript Engine
Labels:
None
Environment:
Windows 10 22H2, Qt Creator 14.0.2

Commits:
032ecd52f (dev), 884b3c575 (6.9), e9b829360 (6.8)

Description

Code

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    width: 640
    height: 480
    visible: true

    Component.onCompleted: console.log("Hi")
    Component.onDestruction: console.log("Bye")
}

Outcomes
Sometimes, the warning doesn't appear at all:

11:45:11: Debugging D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe "-qmljsdebugger=port:59836,block,services:DebugMessages,QmlDebugger,V8Debugger,QmlInspector,DebugTranslation" ...
QML Debugger: Waiting for connection on port 59836...
qml: Hi
qml: Bye
11:45:15: Debugging of D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe "-qmljsdebugger=port:59836,block,services:DebugMessages,QmlDebugger,V8Debugger,QmlInspector,DebugTranslation" has finished with exit code 0.

Sometimes, it appears after the app quits:

11:33:08: Starting D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe...
qml: Hi
qml: Bye
QML debugging is enabled. Only use this in a safe environment.
11:33:10: D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe exited with code 0

Fix
If I add fflush(stderr); to the start of my main() function, then the warning appears at startup:

11:48:04: Debugging D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe "-qmljsdebugger=port:64112,block,services:DebugMessages,QmlDebugger,V8Debugger,QmlInspector,DebugTranslation" ...
QML debugging is enabled. Only use this in a safe environment.
QML Debugger: Waiting for connection on port 64112...
qml: Hi
qml: Bye
11:48:07: Debugging of D:\Test\untitled5\build\Desktop_Qt_6_8_1_MinGW_64_bit-Debug\debug\untitled5.exe "-qmljsdebugger=port:64112,block,services:DebugMessages,QmlDebugger,V8Debugger,QmlInspector,DebugTranslation" has finished with exit code 0.

It looks like simply printing a newline character is not enough to flush the contents of stderr to Qt Creator's Application Output pane: https://codereview.qt-project.org/c/qt/qtdeclarative/+/251021/