Some font scaling factors (like 1.4) are ignored

There are two problems.

Qt scales interface with scaling factor * text scaling factor on X11. I've raised that at https://bugreports.qt.io/browse/QTBUG-84082?focusedCommentId=587793&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-587793, but got no response unfortunately. The fix as simple as follows, but needs to be applied by Qt's xcb platform code:

diff --git a/src/plugins/platforms/xcb/qxcbscreen.cpp b/src/plugins/platforms/xcb/qxcbscreen.cpp
index bb33f393ec..5ecf730f6c 100644
--- a/src/plugins/platforms/xcb/qxcbscreen.cpp
+++ b/src/plugins/platforms/xcb/qxcbscreen.cpp
@@ -77,15 +77,19 @@ QXcbVirtualDesktop::QXcbVirtualDesktop(QXcbConnection *connection, xcb_screen_t
         xcb_depth_next(&depth_iterator);
     }

-    auto dpiChangedCallback = [](QXcbVirtualDesktop *desktop, const QByteArray &, const QVariant &property, void *) {
-        if (!desktop->setDpiFromXSettings(property))
+    auto dpiChangedCallback = [](QXcbVirtualDesktop *desktop, const QByteArray &name, const QVariant &property, void *) {
+        const auto dpi = desktop->dpiFromXSettings(property);
+        auto &oldDpi = name == "Gdk/UnscaledDPI"_L1 ? desktop->m_forcedBaseDpi : desktop->m_forcedDpi;
+        if (dpi <= 0 || oldDpi == dpi)
             return;
-        const auto dpi = desktop->forcedDpi();
+        oldDpi = dpi;
         for (QXcbScreen *screen : desktop->connection()->screens())
-            QWindowSystemInterface::handleScreenLogicalDotsPerInchChange(screen->QPlatformScreen::screen(), dpi, dpi);
+            QWindowSystemInterface::handleScreenLogicalDotsPerInchChange(screen->QPlatformScreen::screen(), desktop->m_forcedDpi, desktop->m_forcedDpi);
     };
-    setDpiFromXSettings(xSettings()->setting("Xft/DPI"));
+    m_forcedDpi = dpiFromXSettings(xSettings()->setting("Xft/DPI"));
+    m_forcedBaseDpi = dpiFromXSettings(xSettings()->setting("Gdk/UnscaledDPI"));
     xSettings()->registerCallbackForProperty("Xft/DPI", dpiChangedCallback, nullptr);
+    xSettings()->registerCallbackForProperty("Gdk/UnscaledDPI", dpiChangedCallback, nullptr);
 }

 QXcbVirtualDesktop::~QXcbVirtualDesktop()
@@ -385,17 +389,13 @@ void QXcbVirtualDesktop::readXResources()
     }
 }

-bool QXcbVirtualDesktop::setDpiFromXSettings(const QVariant &property)
+int QXcbVirtualDesktop::dpiFromXSettings(const QVariant &property)
 {
     bool ok;
     int dpiTimes1k = property.toInt(&ok);
     if (!ok)
-        return false;
-    int dpi = dpiTimes1k / 1024;
-    if (m_forcedDpi == dpi)
-        return false;
-    m_forcedDpi = dpi;
-    return true;
+        return -1;
+    return dpiTimes1k / 1024;
 }

 QSurfaceFormat QXcbVirtualDesktop::surfaceFormatFor(const QSurfaceFormat &format) const
@@ -867,6 +867,15 @@ QDpi QXcbScreen::logicalDpi() const
     return QDpi(96, 96);
 }

+QDpi QXcbScreen::logicalBaseDpi() const
+{
+    const int dpi = forcedBaseDpi();
+    if (dpi > 0)
+        return QDpi(dpi, dpi);
+
+    return QDpi(96, 96);
+}
+
 QPlatformCursor *QXcbScreen::cursor() const
 {
     return m_cursor;
diff --git a/src/plugins/platforms/xcb/qxcbscreen.h b/src/plugins/platforms/xcb/qxcbscreen.h
index 12c13ef80e..ba6147c858 100644
--- a/src/plugins/platforms/xcb/qxcbscreen.h
+++ b/src/plugins/platforms/xcb/qxcbscreen.h
@@ -60,6 +60,7 @@ public:
     void handleScreenChange(xcb_randr_screen_change_notify_event_t *change_event);

     int forcedDpi() const { return m_forcedDpi; }
+    int forcedBaseDpi() const { return m_forcedBaseDpi; }
     QFontEngine::HintStyle hintStyle() const { return m_hintStyle; }
     QFontEngine::SubpixelAntialiasingType subpixelType() const { return m_subpixelType; }
     int antialiasingEnabled() const { return m_antialiasingEnabled; }
@@ -81,7 +82,7 @@ private:
                           QByteArray &stringValue);
     void readXResources();

-    bool setDpiFromXSettings(const QVariant &property);
+    int dpiFromXSettings(const QVariant &property);

     xcb_screen_t *m_screen;
     const int m_number;
@@ -94,6 +95,7 @@ private:
     QRect m_workArea;

     int m_forcedDpi = -1;
+    int m_forcedBaseDpi = -1;
     QFontEngine::HintStyle m_hintStyle = QFontEngine::HintStyle(-1);
     QFontEngine::SubpixelAntialiasingType m_subpixelType = QFontEngine::SubpixelAntialiasingType(-1);
     int m_antialiasingEnabled = -1;
@@ -134,7 +136,7 @@ public:
     QColorSpace colorSpace() const override { return m_colorSpace; }
     QSizeF physicalSize() const override { return m_sizeMillimeters; }
     QDpi logicalDpi() const override;
-    QDpi logicalBaseDpi() const override { return QDpi(96, 96); }
+    QDpi logicalBaseDpi() const override;
     QPlatformCursor *cursor() const override;
     qreal refreshRate() const override { return m_refreshRate; }
     Qt::ScreenOrientation orientation() const override { return m_orientation; }

I'm not sure whethe it's something acceptable though as it reads a gtk-specific setting.

There's no such problem on Wayland, so the previous logic could be applied on wayland, but using manual point-to-pixel conversion: std::round(std::floor(fontSize * 96. / 72. * 100. + 0.5) / 100.). pango_font_description_get_size_is_absolute should say whether such conversion is needed. The result could be multiplied by text scaling factor and applied to QFont::setPixelSize.

FedoraQt / QGnomePlatform

Some font scaling factors (like 1.4) are ignored #44