Qt6.9: Black background / visual glitches when opening some QML dialogs

[krasko78]

Issue type

UI bug (incorrect info or interface appearance)

Description with steps to reproduce

1. Make sure you are using Qt6.9.
2. Open the Export dialog.

Actual: Dialog appears with visible black artefacts for a moment - mostly on the right and bottom edges.

3. Make a change to the score and click the X button to close it. The "Do you want to save the changes?" dialog appears also with black glitches.
4. Open Add -> Insert measures dialog. Same black glitches.

BUT:
5. Open the Parts dialog. No glitches.
6. Open the Preferences dialog. No glitches.

Video:

black_glitches_on_open.mp4

Investigation:
Has to do with non-zero margins set on the dialog. See StyledDialogView.qml and its margins. Dialogs like Parts and Preferences that do not set margins on the dialog (but achieve them by setting them on the inner components instead) work. I also found that moving

`QQuickItem* item = m_view->rootObject();`
`updateSize(QSize(item->implicitWidth(), item->implicitHeight()));`

before

`m_view->show();`

in PopupWindow_QQuickView::show solves the issue.

The Standard Dialogs
The Standard Dialogs, e.g. the "Do you want to save the changes" are a different beast. They resize on the fly, as they are shown, so initially they are shown smaller, and as they are laid out, the implicit sizes are updated, the dialog is also sized up. This is all visible to the user and accompanied by the mysterious black background that Qt6.9 now exhibits.

Supporting files, videos and screenshots

n/a

What is the latest version of MuseScore Studio where this issue is present?

master with Qt6.9

Regression

No.

Operating system

Windows 10

Additional context

No response

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible