Compute logical DPI on a per-screen basis

The logical DPI reported to applications is the platform screen
logical DPI divided by the platform screen scale factor.

Use the screen in question when calculating the DPI instead of
the values from the main screen.

QHighDpiScaling::logicalDpi now takes a QScreen pointer.

Done-with: Friedemann Kleint <Friedemann.Kleint@qt.io>
Task-number: QTBUG-53022
Change-Id: I0f62b5878c37e3488e9a8cc48aef183ff822d0c4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 9 insertions, 11 deletions

diff --git a/src/gui/kernel/qhighdpiscaling.cpp b/src/gui/kernel/qhighdpiscaling.cpp
index 64f1397771..95790b96a9 100644
--- a/src/gui/kernel/qhighdpiscaling.cpp
+++ b/src/gui/kernel/qhighdpiscaling.cpp
@@ -226,7 +226,6 @@ bool QHighDpiScaling::m_usePixelDensity = false; // use scale factor from platfo
 bool QHighDpiScaling::m_pixelDensityScalingActive = false; // pixel density scale factor > 1
 bool QHighDpiScaling::m_globalScalingActive = false; // global scale factor is active
 bool QHighDpiScaling::m_screenFactorSet = false; // QHighDpiScaling::setScreenFactor has been used
-QDpi QHighDpiScaling::m_logicalDpi = QDpi(-1,-1); // The scaled logical DPI of the primary screen
 
 /*
     Initializes the QHighDpiScaling global variables. Called before the
@@ -314,14 +313,6 @@ void QHighDpiScaling::updateHighDpiScaling()
         }
     }
     m_active = m_globalScalingActive || m_screenFactorSet || m_pixelDensityScalingActive;
-
-    QScreen *primaryScreen = QGuiApplication::primaryScreen();
-    if (!primaryScreen)
-        return;
-    QPlatformScreen *platformScreen = primaryScreen->handle();
-    qreal sf = screenSubfactor(platformScreen);
-    QDpi primaryDpi = platformScreen->logicalDpi();
-    m_logicalDpi = QDpi(primaryDpi.first / sf, primaryDpi.second / sf);
 }
 
 /*
@@ -447,9 +438,16 @@ qreal QHighDpiScaling::screenSubfactor(const QPlatformScreen *screen)
     return factor;
 }
 
-QDpi QHighDpiScaling::logicalDpi()
+QDpi QHighDpiScaling::logicalDpi(const QScreen *screen)
 {
-    return m_logicalDpi;
+    // (Note: m_active test is performed at call site.)
+    if (!screen)
+        return QDpi(96, 96);
+
+    qreal platformScreenfactor = screenSubfactor(screen->handle());
+    QDpi platformScreenDpi = screen->handle()->logicalDpi();
+    return QDpi(platformScreenDpi.first / platformScreenfactor,
+                platformScreenDpi.second / platformScreenfactor);
 }
 
 QHighDpiScaling::ScaleAndOrigin QHighDpiScaling::scaleAndOrigin(const QPlatformScreen *platformScreen, QPoint *nativePosition)