Windows QPA: fix window style correction for context help button

According to Microsoft Docs [1][2], WS_MINIMIZEBOX and WS_MAXIMIZEBOX
must be accompanied by the WS_SYSMENU style, and the WS_EX_CONTEXTHELP
style is not compatible with WS_MINIMIZEBOX and WS_MAXIMIZEBOX. This
patch adds additional checks for these situations.

[1] https://docs.microsoft.com/en-us/windows/win32/winmsg/window-styles
[2] https://docs.microsoft.com/en-us/windows/win32/winmsg/extended-window-styles

Pick-to: 6.3 6.2
Change-Id: If32f8b42e25cfc67ffd1e84cc4b061f21a01042a
Reviewed-by: André de la Rocha <andre.rocha@qt.io>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 1594a4057a..d28a3ee675 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -853,13 +853,18 @@ void WindowCreationData::fromWindow(const QWindow *w, const Qt::WindowFlags flag
                     style |= WS_SYSMENU | WS_BORDER; // QTBUG-2027, dialogs without system menu.
                     exStyle |= WS_EX_DLGMODALFRAME;
                 }
-                if (flags & Qt::WindowMinimizeButtonHint)
+                const bool showMinimizeButton = flags & Qt::WindowMinimizeButtonHint;
+                if (showMinimizeButton)
                     style |= WS_MINIMIZEBOX;
-                if (shouldShowMaximizeButton(w, flags))
+                const bool showMaximizeButton = shouldShowMaximizeButton(w, flags);
+                if (showMaximizeButton)
                     style |= WS_MAXIMIZEBOX;
+                if (showMinimizeButton || showMaximizeButton)
+                    style |= WS_SYSMENU;
                 if (tool)
                     exStyle |= WS_EX_TOOLWINDOW;
-                if (flags & Qt::WindowContextHelpButtonHint)
+                if ((flags & Qt::WindowContextHelpButtonHint) && !showMinimizeButton
+                    && !showMaximizeButton)
                     exStyle |= WS_EX_CONTEXTHELP;
             } else {
                  exStyle |= WS_EX_TOOLWINDOW;