QWindowsWindow: Work around offset bug in GetWindowPlacement()

GetWindowPlacement() stores wrong positions in the &WINDOWPLACEMENT
argument's ptMaxPosition member on Windows 10.

(1) Each time a fixed height window is maximized, ptMaxPosition moves
- down by the task bar's height, if the task bar is on top
- right by the task bar's width, if the task bar is on the left

(2) The frame margins' left offset (typically -8) is not respected.

On Windows 10, set ptMaxPosition to {0, 0} adjusted by the frame
margins' left offset.

Fixes: QTBUG-123752
Pick-to: 6.7 6.5 6.2
Change-Id: I8bbd63d93a7648921b703ff6f3e09c4cf2999be2
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
(cherry picked from commit 33874a9424cd03c21ca9986a39c383b3689f03d2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -36,6 +36,7 @@
 
 #include <QtCore/qdebug.h>
 #include <QtCore/qlibraryinfo.h>
+#include <QtCore/qoperatingsystemversion.h>
 
 #include <dwmapi.h>
 
@@ -2462,6 +2463,12 @@ QWindowsWindowData QWindowsWindow::setWindowFlags_sys(Qt::WindowFlags wt,
     return result;
 }
 
+inline bool QWindowsBaseWindow::hasMaximumSize() const
+{
+    const auto maximumSize = window()->maximumSize();
+    return maximumSize.width() != QWINDOWSIZE_MAX || maximumSize.height() != QWINDOWSIZE_MAX;
+}
+
 void QWindowsWindow::handleWindowStateChange(Qt::WindowStates state)
 {
     qCDebug(lcQpaWindow) << __FUNCTION__ << this << window()
@@ -2478,6 +2485,21 @@ void QWindowsWindow::handleWindowStateChange(Qt::WindowStates state)
             GetWindowPlacement(m_data.hwnd, &windowPlacement);
             const RECT geometry = RECTfromQRect(m_data.restoreGeometry);
             windowPlacement.rcNormalPosition = geometry;
+
+            // A bug in windows 10 grows
+            // - ptMaxPosition.x by the task bar's width, if it's on the left
+            // - ptMaxPosition.y by the task bar's height, if it's on the top
+            // each time GetWindowPlacement() is called.
+            // The offset of the screen's left edge (as per frameMargins_sys().left()) is ignored.
+            // => Check for windows 10 and correct.
+            static const auto windows11 = QOperatingSystemVersion::Windows11_21H2;
+            static const bool isWindows10 = QOperatingSystemVersion::current() < windows11;
+            if (isWindows10 && hasMaximumSize()) {
+                const QMargins margins = frameMargins_sys();
+                const QPoint topLeft = window()->screen()->geometry().topLeft();
+                windowPlacement.ptMaxPosition = POINT{ topLeft.x() - margins.left(), topLeft.y() };
+            }
+
             // Even if the window is hidden, windowPlacement's showCmd is not SW_HIDE, so change it
             // manually to avoid unhiding a hidden window with the subsequent call to
             // SetWindowPlacement().

src/plugins/platforms/windows/qwindowswindow.h

@@ -128,6 +128,7 @@ public:
 protected:
     HWND parentHwnd() const { return GetAncestor(handle(), GA_PARENT); }
     bool isTopLevel_sys() const;
+    inline bool hasMaximumSize() const;
     QRect frameGeometry_sys() const;
     QRect geometry_sys() const;
     void setGeometry_sys(const QRect &rect) const;