tdf#160565 tdf#145735 qt: Avoid native window handles for Qt 6

Similar to the tdf#122293 scenario with Qt 5 on Wayland,
using native windows causes unresponsive UI even with
the xcb Qt QPA plugin when using Qt 6.

Therefore, don't call `QWidget::winId()` at all when
using Qt 6, but just refuse to resolve a native window
handle, the same way that is already done here for the
wayland Qt QPA plugin with qt5.

Add a comment based on the one originally added with

commit 0e3c3b842e14b9646d3697cf1266be21359e0f13
Author: Michael Weghorn <m.weghorn@posteo.de>
Date:   Sat May 11 21:31:33 2019 +0200

tdf#122293 qt5: Use "alien widgets" by default on Wayland

that was later dropped during a refactoring.

This code path is triggered when trying to resolve the
parent window for a file picker (s. `QtFilePicker::initialize`).
In a quick test in a KDE Plasma 5 Wayland session on Debian testing,
the modality behavior was unchanged with or without this change in
place:

* With qt6 xcb, the main window nicely gets grayed out and is
unresponsive while the file dialog gets shown  when using
e.g. "File" -> "Open" in Writer.

* With qt6 wayland, the main window is not grayed out and looks
more "active" still, but doesn't actually accept keyboard
or mouse input.

This is basically the same in other Qt/KDE applications, e.g.
a self-compiled Kate from git master, so not LO-specific.

If resolving the parent for the file picker turns out to be necessary,
maybe something similar to what commit

commit 3ff4800fe400de916c97f587322104af06cc0879
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Wed Apr 4 10:22:34 2018 +0100

weld SvInsertOleDlg

and

weld SfxInsertFloatingFrameDialog

and smuggle in the parent widget for the Gtk File dialog via
an XWindow interface

did for gtk3 might be worth looking into.

However, I can at least not reproduce issues like the one described
for gtk3 in

commit 203d96787969f707c78101be18d51b44ace98f93
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Mon Jun 21 13:01:52 2021 +0100

give folderpicker an optional parent

so, like a file picker, it can make its parent modal while its
operating. Otherwise its possible to interact with the parent dialog in
undesirable ways, e.g. file, export as, export as epub, the folder
picker of 'media directory'

with the qt6 VCL plugin even with this commit in place.

Another scenario relying on a native window handle is video
playback with GStreamer's x11 video sink.

That still works for qt5 with the xcb plugin as the handle is
still returned for that one.

For qt6 with the xcb plugin, that didn't work properly
without this commit either (at least not in my tests with
current qtbase dev as of commit
70a2e7f32b9f9ce19d1538f14fbde7b0d1e77ffd), s.
tdf#145735 comment 7.
It's now broken a different way than before
(extra windows show up instead of no video being shown).
This will be further tracked together with a solution for
Wayland in tdf#125219.

As a side note, forcing native windows for everything
using `QT_USE_NATIVE_WINDOWS=1` as described at [1]
causes more sever brokenness with the qt6 xcb plugin
(mostly black window in Writer instead of showing the
actual content).

[1] https://doc.qt.io/qt-6/qwidget.html#native-widgets-vs-alien-widgets

Change-Id: I9718c680bd8bc4ff0574f171403d965c1beac781
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167783
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins

1 files changed, 11 insertions, 1 deletions

diff --git a/vcl/qt5/QtFrame.cxx b/vcl/qt5/QtFrame.cxx
index 0ff940408769..0950b520cb77 100644
--- a/vcl/qt5/QtFrame.cxx
+++ b/vcl/qt5/QtFrame.cxx
@@ -35,6 +35,7 @@
 #include <QtX11Support.hxx>
 #endif
 
+#include <QtCore/QLibraryInfo>
 #include <QtCore/QMimeData>
 #include <QtCore/QPoint>
 #include <QtCore/QSize>
@@ -1355,8 +1356,17 @@ void QtFrame::ResolveWindowHandle(SystemEnvData& rData) const
     if (!rData.pWidget)
         return;
     assert(rData.platform != SystemEnvData::Platform::Invalid);
-    if (rData.platform != SystemEnvData::Platform::Wayland)
+    // Calling QWidget::winId() implicitly enables native windows to be used instead
+    // of "alien widgets" that don't have a native widget asscoiated with them,
+    // s. https://doc.qt.io/qt-6/qwidget.html#native-widgets-vs-alien-widgets
+    // Avoid native widgets with Qt 5 on Wayland and with Qt 6 altogether as they
+    // cause unsresponsive UI, s. tdf#122293/QTBUG-75766 and tdf#160565
+    // (for qt5 xcb, they're needed for video playback)
+    if (rData.platform != SystemEnvData::Platform::Wayland
+        && QLibraryInfo::version().majorVersion() < 6)
+    {
         rData.SetWindowHandle(static_cast<QWidget*>(rData.pWidget)->winId());
+    }
 }
 
 bool QtFrame::GetUseReducedAnimation() const { return GetQtInstance()->GetUseReducedAnimation(); }