Fix QWindow::mapToGlobal()/mapFromGlobal() for multi-screen windows

Make these functions handle the case where a window spans multiple
screens, and high-DPI scaling is enabled, and the local position (in the
window) is not on the window primary screen (as returned by
QWindow::screen()).

This is done by detecting the case, and then calculating
the correct position using the native coordinate system.

[ChangeLog][QtGui] QWindow::mapToGlobal()/mapFromGlobal() now handle
windows spanning screens correctly.

Done-with: Friedemann Kleint<Friedemann.Kleint@qt.io>
Task-number: QTBUG-73231
Change-Id: I3c31b741344d9e85e4f5d9e60bae75acce2db741
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qhighdpiscaling.cpp

@@ -41,7 +41,9 @@
 #include "qguiapplication.h"
 #include "qscreen.h"
 #include "qplatformintegration.h"
+#include "qplatformwindow.h"
 #include "private/qscreen_p.h"
+#include <private/qguiapplication_p.h>
 
 #include <QtCore/qdebug.h>
 
@@ -376,6 +378,46 @@ QPoint QHighDpiScaling::mapPositionFromNative(const QPoint &pos, const QPlatform
     return (pos - topLeft) / scaleFactor + topLeft;
 }
 
+QPoint QHighDpiScaling::mapPositionToGlobal(const QPoint &pos, const QPoint &windowGlobalPosition, const QWindow *window)
+{
+    QPoint globalPosCandidate = pos + windowGlobalPosition;
+    if (QGuiApplicationPrivate::screen_list.size() <= 1)
+        return globalPosCandidate;
+
+    // The global position may be outside device independent screen geometry
+    // in cases where a window spans screens. Detect this case and map via
+    // native coordinates to the correct screen.
+    auto currentScreen = window->screen();
+    if (currentScreen && !currentScreen->geometry().contains(globalPosCandidate)) {
+        auto nativeGlobalPos = QHighDpi::toNativePixels(globalPosCandidate, currentScreen);
+        if (auto actualPlatformScreen = currentScreen->handle()->screenForPosition(nativeGlobalPos))
+            return QHighDpi::fromNativePixels(nativeGlobalPos, actualPlatformScreen->screen());
+    }
+
+    return globalPosCandidate;
+}
+
+QPoint QHighDpiScaling::mapPositionFromGlobal(const QPoint &pos, const QPoint &windowGlobalPosition, const QWindow *window)
+{
+    QPoint windowPosCandidate = pos - windowGlobalPosition;
+    if (QGuiApplicationPrivate::screen_list.size() <= 1)
+        return windowPosCandidate;
+
+    // Device independent global (screen) space may discontiguous when high-dpi scaling
+    // is active. This means that the normal subtracting of the window global position from the
+    // position-to-be-mapped may not work in cases where a window spans multiple screens.
+    // Map both positions to native global space (using the correct screens), subtract there,
+    // and then map the difference back using the scale factor for the window.
+    QScreen *posScreen = QGuiApplication::screenAt(pos);
+    if (posScreen && posScreen != window->screen()) {
+        QPoint nativePos = QHighDpi::toNativePixels(pos, posScreen);
+        QPoint windowNativePos = window->handle()->geometry().topLeft();
+        return QHighDpi::fromNativeLocalPosition(nativePos - windowNativePos, window);
+    }
+
+    return windowPosCandidate;
+}
+
 qreal QHighDpiScaling::screenSubfactor(const QPlatformScreen *screen)
 {
     qreal factor = qreal(1.0);

src/gui/kernel/qhighdpiscaling_p.h

@@ -83,8 +83,10 @@ public:
     static qreal factor(const QPlatformScreen *platformScreen);
     static QPoint origin(const QScreen *screen);
     static QPoint origin(const QPlatformScreen *platformScreen);
-    static QPoint mapPositionFromNative(const QPoint &pos, const QPlatformScreen *platformScreen);
     static QPoint mapPositionToNative(const QPoint &pos, const QPlatformScreen *platformScreen);
+    static QPoint mapPositionFromNative(const QPoint &pos, const QPlatformScreen *platformScreen);
+    static QPoint mapPositionToGlobal(const QPoint &pos, const QPoint &windowGlobalPosition, const QWindow *window);
+    static QPoint mapPositionFromGlobal(const QPoint &pos, const QPoint &windowGlobalPosition, const QWindow *window);
     static QDpi logicalDpi();
 
 private:

src/gui/kernel/qwindow.cpp

@@ -1670,9 +1670,9 @@ void QWindow::setGeometry(const QRect &rect)
   chicken and egg problem here: we cannot convert to native coordinates
   before we know which screen we are on.
 */
-QScreen *QWindowPrivate::screenForGeometry(const QRect &newGeometry)
+QScreen *QWindowPrivate::screenForGeometry(const QRect &newGeometry) const
 {
-    Q_Q(QWindow);
+    Q_Q(const QWindow);
     QScreen *currentScreen = q->screen();
     QScreen *fallback = currentScreen;
     QPoint center = newGeometry.center();
@@ -2542,6 +2542,10 @@ QPoint QWindow::mapToGlobal(const QPoint &pos) const
         && (d->platformWindow->isForeignWindow() || d->platformWindow->isEmbedded())) {
         return QHighDpi::fromNativeLocalPosition(d->platformWindow->mapToGlobal(QHighDpi::toNativeLocalPosition(pos, this)), this);
     }
+
+    if (QHighDpiScaling::isActive())
+        return QHighDpiScaling::mapPositionToGlobal(pos, d->globalPosition(), this);
+
     return pos + d->globalPosition();
 }
 
@@ -2562,6 +2566,10 @@ QPoint QWindow::mapFromGlobal(const QPoint &pos) const
         && (d->platformWindow->isForeignWindow() || d->platformWindow->isEmbedded())) {
         return QHighDpi::fromNativeLocalPosition(d->platformWindow->mapFromGlobal(QHighDpi::toNativeLocalPosition(pos, this)), this);
     }
+
+    if (QHighDpiScaling::isActive())
+        return QHighDpiScaling::mapPositionFromGlobal(pos, d->globalPosition(), this);
+
     return pos - d->globalPosition();
 }
 

src/gui/kernel/qwindow_p.h

@@ -148,7 +148,7 @@ public:
     void connectToScreen(QScreen *topLevelScreen);
     void disconnectFromScreen();
     void emitScreenChangedRecursion(QScreen *newScreen);
-    QScreen *screenForGeometry(const QRect &rect);
+    QScreen *screenForGeometry(const QRect &rect) const;
 
     virtual void clearFocusObject();
     virtual QRectF closestAcceptableGeometry(const QRectF &rect) const;