tdf#159915 qt: Force Qt::HighDpiScaleFactorRoundingPolicy::Round

For now, force `Qt::HighDpiScaleFactorRoundingPolicy::Round`
for the HighDPI-scale factor rounding policy [1], which is the default
for Qt 5, while Qt 6 defaults to `Qt::HighDpiScaleFactorRoundingPolicy::PassThrough`
(see [2]), which resulted in broken rendering (e.g. "Help" -> "About"
dialog not showing the whole content) when fractional display scaling like 150 %
is configured in the KDE Plasma display settings (in contrast to manually setting the
`QT_SCALE_FACTOR=1.5` env variable to apply scaling, which was working
fine).

Quoting from [3]:

> The two principal options are whether fractional scale factors should be
> rounded to an integer or not. Keeping the scale factor as-is will make
> the user interface size match the OS setting exactly, but may cause
> painting errors, for example with the Windows style.

Manually setting the env variable `QT_SCALE_FACTOR_ROUNDING_POLICY="Round"`
has the same effect (and can be used with LO versions not yet
containing this fix).

(There might be a way to adjust the way that scaling happens
to make other policies work, but for now, just hard-code to
the policy that is known to work.)

[1] https://doc.qt.io/qt-6/qt.html#HighDpiScaleFactorRoundingPolicy-enum
[2] https://doc.qt.io/qt-6/highdpi.html#environment-variable-reference
[3] https://doc.qt.io/qt-6/qguiapplication.html#setHighDpiScaleFactorRoundingPolicy

Change-Id: I8eb6911d4dd5faf00912b8f15a58e0bdace1995a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/164768
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
(cherry picked from commit 07688e864c913e005dcae366cf10702404a73d80)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/164744
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/vcl/qt5/QtInstance.cxx b/vcl/qt5/QtInstance.cxx
index 4880c1bdec55..f87cdef9348e 100644
--- a/vcl/qt5/QtInstance.cxx
+++ b/vcl/qt5/QtInstance.cxx
@@ -708,6 +708,11 @@ std::unique_ptr<QApplication> QtInstance::CreateQApplication(int& nArgc, char**
     // for scaled icons in the native menus
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
 #endif
+    // force Qt::HighDpiScaleFactorRoundingPolicy::Round, which is the Qt 5 default
+    // policy and prevents incorrect rendering with the Qt 6 default policy
+    // Qt::HighDpiScaleFactorRoundingPolicy::PassThrough (tdf#159915)
+    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(
+        Qt::HighDpiScaleFactorRoundingPolicy::Round);
 
     FreeableCStr session_manager;
     if (getenv("SESSION_MANAGER") != nullptr)