Windows: Add support for invisible window margins

Windows 10 windows contain an invisible area within the NC window frame
on which the mouse cursor is enabled to perform resizing. This change
captures the geometry of the invisible margins and considers it when
moving a window, so that, for instance, a move(0,0) does not generate
gap between the window and the beginning of screen.

[ChangeLog][Windows] The dimensions of invisible margins inside the
frames of Windows 10 windows will now be disregarded in the positioning
of Qt windows to avoid a misplaced look (offset by a few pixels from
the expected position).

Task-number: QTBUG-55762
Change-Id: I1f537756eb1a093f78b919de9d44992528199700
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

4 files changed, 60 insertions, 21 deletions

diff --git a/src/plugins/platforms/windows/qwindowscontext.cpp b/src/plugins/platforms/windows/qwindowscontext.cpp
index 2def926409..c5146aac84 100644
--- a/src/plugins/platforms/windows/qwindowscontext.cpp
+++ b/src/plugins/platforms/windows/qwindowscontext.cpp
@@ -1401,7 +1401,7 @@ extern "C" LRESULT QT_WIN_CALLBACK qWindowsWndProc(HWND hwnd, UINT message, WPAR
             marginsFromRects(ncCalcSizeFrame, rectFromNcCalcSize(message, wParam, lParam, 0));
         if (margins.left() >= 0) {
             if (platformWindow) {
-                platformWindow->setFrameMargins(margins);
+                platformWindow->setFullFrameMargins(margins);
             } else {
                 const QSharedPointer<QWindowCreationContext> ctx = QWindowsContext::instance()->windowCreationContext();
                 if (!ctx.isNull())
diff --git a/src/plugins/platforms/windows/qwindowsintegration.cpp b/src/plugins/platforms/windows/qwindowsintegration.cpp
index 287b65cd5d..fc85f5199c 100644
--- a/src/plugins/platforms/windows/qwindowsintegration.cpp
+++ b/src/plugins/platforms/windows/qwindowsintegration.cpp
@@ -332,7 +332,7 @@ QPlatformWindow *QWindowsIntegration::createPlatformWindow(QWindow *window) cons
         << "\n    Requested: " << requested.geometry << " frame incl.="
         << QWindowsGeometryHint::positionIncludesFrame(window)
         << ' ' << requested.flags
-        << "\n    Obtained : " << obtained.geometry << " margins=" << obtained.frame
+        << "\n    Obtained : " << obtained.geometry << " margins=" << obtained.fullFrameMargins
         << " handle=" << obtained.hwnd << ' ' << obtained.flags << '\n';
 
     if (Q_UNLIKELY(!obtained.hwnd))
diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 552c4c0f5c..81af784a75 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -422,6 +422,31 @@ static inline void updateGLWindowSettings(const QWindow *w, HWND hwnd, Qt::Windo
 }
 
 /*!
+    Calculates the dimensions of the invisible borders within the
+    window frames in Windows 10, using an empirical expression that
+    reproduces the measured values for standard DPI settings.
+*/
+
+static QMargins invisibleMargins(QPoint screenPoint)
+{
+    if (QOperatingSystemVersion::current() >= QOperatingSystemVersion::Windows10) {
+        POINT pt = {screenPoint.x(), screenPoint.y()};
+        if (HMONITOR hMonitor = MonitorFromPoint(pt, MONITOR_DEFAULTTONULL)) {
+            if (QWindowsContext::shcoredll.isValid()) {
+                UINT dpiX;
+                UINT dpiY;
+                if (SUCCEEDED(QWindowsContext::shcoredll.getDpiForMonitor(hMonitor, 0, &dpiX, &dpiY))) {
+                    const qreal sc = (dpiX - 96) / 96.0;
+                    const int gap = 7 + qRound(5*sc) - int(sc);
+                    return QMargins(gap, 0, gap, gap);
+                }
+            }
+        }
+    }
+    return QMargins();
+}
+
+/*!
     \class WindowCreationData
     \brief Window creation code.
 
@@ -651,16 +676,20 @@ QWindowsWindowData
     const QWindowCreationContextPtr context(new QWindowCreationContext(w, data.geometry, rect, data.customMargins, style, exStyle));
     QWindowsContext::instance()->setWindowCreationContext(context);
 
+    QMargins invMargins = topLevel && !(result.flags & Qt::FramelessWindowHint) && QWindowsGeometryHint::positionIncludesFrame(w)
+            ? invisibleMargins(QPoint(context->frameX, context->frameY)) : QMargins();
+
     qCDebug(lcQpaWindows).nospace()
         << "CreateWindowEx: " << w << " class=" << windowClassName << " title=" << title
         << '\n' << *this << "\nrequested: " << rect << ": "
         << context->frameWidth << 'x' <<  context->frameHeight
         << '+' << context->frameX << '+' << context->frameY
-        << " custom margins: " << context->customMargins;
+        << " custom margins: " << context->customMargins
+        << " invisible margins: " << invMargins;
 
     result.hwnd = CreateWindowEx(exStyle, classNameUtf16, titleUtf16,
                                  style,
-                                 context->frameX, context->frameY,
+                                 context->frameX - invMargins.left(), context->frameY - invMargins.top(),
                                  context->frameWidth, context->frameHeight,
                                  parentHandle, NULL, appinst, NULL);
     qCDebug(lcQpaWindows).nospace()
@@ -673,7 +702,7 @@ QWindowsWindowData
     }
 
     result.geometry = context->obtainedGeometry;
-    result.frame = context->margins;
+    result.fullFrameMargins = context->margins;
     result.embedded = embedded;
     result.customMargins = context->customMargins;
 
@@ -887,7 +916,7 @@ QRect QWindowsBaseWindow::frameGeometry_sys() const
 
 QRect QWindowsBaseWindow::geometry_sys() const
 {
-    return frameGeometry_sys().marginsRemoved(frameMargins());
+    return frameGeometry_sys().marginsRemoved(fullFrameMargins());
 }
 
 QMargins QWindowsBaseWindow::frameMargins_sys() const
@@ -1560,7 +1589,7 @@ QRect QWindowsWindow::normalGeometry() const
     const bool fakeFullScreen =
         m_savedFrameGeometry.isValid() && (window()->windowStates() & Qt::WindowFullScreen);
     const QRect frame = fakeFullScreen ? m_savedFrameGeometry : normalFrameGeometry(m_data.hwnd);
-    const QMargins margins = fakeFullScreen ? QWindowsGeometryHint::frame(m_savedStyle, 0) : frameMargins();
+    const QMargins margins = fakeFullScreen ? QWindowsGeometryHint::frame(m_savedStyle, 0) : fullFrameMargins();
     return frame.isValid() ? frame.marginsRemoved(margins) : frame;
 }
 
@@ -1592,8 +1621,8 @@ void QWindowsWindow::setGeometry(const QRect &rectIn)
                      window()->metaObject()->className(), qPrintable(window()->objectName()),
                      m_data.geometry.width(), m_data.geometry.height(),
                      m_data.geometry.x(), m_data.geometry.y(),
-                     m_data.frame.left(), m_data.frame.top(),
-                     m_data.frame.right(), m_data.frame.bottom(),
+                     m_data.fullFrameMargins.left(), m_data.fullFrameMargins.top(),
+                     m_data.fullFrameMargins.right(), m_data.fullFrameMargins.bottom(),
                      m_data.customMargins.left(), m_data.customMargins.top(),
                      m_data.customMargins.right(), m_data.customMargins.bottom(),
                      window()->minimumWidth(), window()->minimumHeight(),
@@ -1685,7 +1714,7 @@ void QWindowsWindow::handleGeometryChange()
 
 void QWindowsBaseWindow::setGeometry_sys(const QRect &rect) const
 {
-    const QMargins margins = frameMargins();
+    const QMargins margins = fullFrameMargins();
     const QRect frameGeometry = rect + margins;
 
     qCDebug(lcQpaWindows) << '>' << __FUNCTION__ << window()
@@ -2106,21 +2135,29 @@ bool QWindowsWindow::handleGeometryChangingMessage(MSG *message, const QWindow *
 
 bool QWindowsWindow::handleGeometryChanging(MSG *message) const
 {
-    const QMargins margins = window()->isTopLevel() ? frameMargins() : QMargins();
+    const QMargins margins = window()->isTopLevel() ? fullFrameMargins() : QMargins();
     return QWindowsWindow::handleGeometryChangingMessage(message, window(), margins);
 }
 
-void QWindowsWindow::setFrameMargins(const QMargins &newMargins)
+void QWindowsWindow::setFullFrameMargins(const QMargins &newMargins)
 {
-    if (m_data.frame != newMargins) {
-        qCDebug(lcQpaWindows) << __FUNCTION__ << window() <<  m_data.frame  << "->" << newMargins;
-        m_data.frame = newMargins;
+    if (m_data.fullFrameMargins != newMargins) {
+        qCDebug(lcQpaWindows) << __FUNCTION__ << window() <<  m_data.fullFrameMargins  << "->" << newMargins;
+        m_data.fullFrameMargins = newMargins;
     }
 }
 
 QMargins QWindowsWindow::frameMargins() const
 {
-    return m_data.frame;
+    QMargins result = fullFrameMargins();
+    if (isTopLevel() && !(m_data.flags & Qt::FramelessWindowHint))
+        result -= invisibleMargins(geometry().topLeft());
+    return result;
+}
+
+QMargins QWindowsWindow::fullFrameMargins() const
+{
+    return m_data.fullFrameMargins;
 }
 
 void QWindowsWindow::setOpacity(qreal level)
@@ -2174,7 +2211,7 @@ void QWindowsWindow::setMask(const QRegion &region)
 
     // Mask is in client area coordinates, so offset it in case we have a frame
     if (window()->isTopLevel()) {
-        const QMargins margins = frameMargins();
+        const QMargins margins = fullFrameMargins();
         OffsetRgn(winRegion, margins.left(), margins.top());
     }
 
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 68e0617b19..6606f4ee7a 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -108,8 +108,8 @@ struct QWindowsWindowData
 {
     Qt::WindowFlags flags;
     QRect geometry;
-    QMargins frame; // Do not use directly for windows, see FrameDirty.
-    QMargins customMargins; // User-defined, additional frame for NCCALCSIZE
+    QMargins fullFrameMargins; // Do not use directly for windows, see FrameDirty.
+    QMargins customMargins;    // User-defined, additional frame for NCCALCSIZE
     HWND hwnd = 0;
     bool embedded = false;
 
@@ -125,9 +125,10 @@ public:
 
     WId winId() const override { return WId(handle()); }
     QRect geometry() const override { return geometry_sys(); }
-    QMargins frameMargins() const override { return frameMargins_sys(); }
+    QMargins frameMargins() const override { return fullFrameMargins(); }
     QPoint mapToGlobal(const QPoint &pos) const override;
     QPoint mapFromGlobal(const QPoint &pos) const override;
+    virtual QMargins fullFrameMargins() const { return frameMargins_sys(); }
 
     using QPlatformWindow::screenForGeometry;
 
@@ -258,7 +259,8 @@ public:
     static bool handleGeometryChangingMessage(MSG *message, const QWindow *qWindow, const QMargins &marginsDp);
     bool handleGeometryChanging(MSG *message) const;
     QMargins frameMargins() const override;
-    void setFrameMargins(const QMargins &newMargins);
+    QMargins fullFrameMargins() const override;
+    void setFullFrameMargins(const QMargins &newMargins);
 
     void setOpacity(qreal level) override;
     void setMask(const QRegion &region) override;