[dolphin] [Bug 419360] New: dolphin show exit confirmation dialog on session logout with qt >= 5.14.0

[i.Dark_Templar]

https://bugs.kde.org/show_bug.cgi?id=419360

            Bug ID: 419360
           Summary: dolphin show exit confirmation dialog on session
                    logout with qt >= 5.14.0
           Product: dolphin
           Version: 19.12.3
          Platform: Gentoo Packages
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: dolphin-bugs-null at kde.org
          Reporter: idarktemplar at mail.ru
                CC: kfm-devel at kde.org
  Target Milestone: ---

Created attachment 127076
  --> https://bugs.kde.org/attachment.cgi?id=127076&action=edit
some information from gdb

After upgrade from Qt-5.13.2 to Qt-5.14.1 on KDE session shutdown dolphin
started asking to confirm if I want to quit while multiple tabs are open. It
didn't ask for such confirmation with Qt-5.13.2 on session logout. And current
session is saved successfully with no regards which button was pressed in this
dialog or if any button was pressed at all.

STEPS TO REPRODUCE
1. Open at least 2 tabs in dolphin
2. Ensure that confirmation to close dolphin window with multiple open tabs is
enabled
3. Ensure that qt-5.14.1 or newer is used
4. Ensure that previous session restoring is selected in session management
configuration of KDE
4. Logout from KDE session

OBSERVED RESULT
Dolphin asks to confirm closing window on session logout. If you wait too long,
it'll just eventually crash, but on next start session would be restored.

EXPECTED RESULT
Dolphin should silently save current session and restore it next time

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Gentoo Linux with kernel 4.14.166
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1
Dolphin Version: 19.12.3

ADDITIONAL INFORMATION
While bug appears in dolphin 19.12.3 for me, I guess it'd appear in other
versions too when Qt >= 5.14.0 is used.

I suspect that it's related to following Qt change but I didn't confirm it:
https://code.qt.io/cgit/qt/qtbase.git/commit/?id=1b6db184947

Attached file contains a bit of information I could obtain using gdb. It looks
like dolphin receives two close events on session logout. In first one
'QGuiApplication::isSavingSession()' returns true and dolphin silently saves
information about current session. In second one
'QGuiApplication::isSavingSession()' returns false and it triggers displaying
confirmation dialog.

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