[Breeze] [Bug 368048] New: Very strange QLineEdit inside QMenu glitch under extreemly specific conditions

[Alex Maystrenko via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=368048

            Bug ID: 368048
           Summary: Very strange QLineEdit inside QMenu glitch under
                    extreemly specific conditions
           Product: Breeze
           Version: 5.7.3
          Platform: Archlinux Packages
               URL: https://github.com/technic/qt5-bug/blob/master/bug.png
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: plasma-devel at kde.org
          Reporter: alexeytech at gmail.com

This strange thing initially happened in LXQt project, you may see discussion
here https://github.com/lxde/lxqt/issues/1109

However, I've figured out that this looks like Qt, Breeze or Xorg bug. I am
posting it here, because in my case it is reproducible only with breeze and
oxygen theme, but not with fusion. Maybe, additional factors like video drivers
are also important.

I created an example, that illustrates the bug:

There is a QWidgetAction based on QLineEdit, that is added to QMenu. There is a
function, that deletes old QMenu instance and creates a new one, but
QWidgetAction is moved to new parent with removeAction/addAction functions.
Everything works fine so far.

But next two lines of code added simultaneously, break widget drawing. We set
background for the root QMenu, and added a child QMenu:
mMenu->setStyleSheet("QMenu { background: rgba(240, 0, 0, 100%); }");
QMenu *submenu = new QMenu(mMenu);

Hmm...?)))



Reproducible: Always

Steps to Reproduce:
1. go to https://github.com/technic/qt5-bug and build the project
2. run it and wait for 3sec (glitch creation is managed by timers)

Actual Results:  
A transparent space instead of QLineEdit (see screenshot)

Expected Results:  
QLineEdit popup:
https://github.com/technic/qt5-bug/blob/master/ok.png

Only breeze and oxygen themes are affected. Fusion theme works fine.

Qt 5.7.0, radeon opensource driver.

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