Add Qt::NoTitleBarBackgroundHint window flag

The flag requests that the window's title bar is drawn without a
background color. This flag is useful in combination with the
Qt::ExpandedClientAreaHint flag, to give the perception that the
window's client area seamlessly blends with the titlebar area
and controls.

Task-number: QTBUG-127634
Change-Id: I1194630d737ae03324f79d2babd7ea3d8fca3d5b
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/corelib/global/qnamespace.h

@@ -240,6 +240,7 @@ namespace Qt {
             "Use Qt::ExpandedClientAreaHint instead") = 0x00400000,
 #endif
         ExpandedClientAreaHint = 0x00400000,
+        NoTitleBarBackgroundHint = 0x00800000,
 
         CustomizeWindowHint = 0x02000000,
         WindowStaysOnBottomHint = 0x04000000,

src/corelib/global/qnamespace.qdoc

@@ -2353,6 +2353,11 @@
            window's \l{QWindow::safeAreaMargins()}{safe area margins} will reflect any
            areas that may have conflicting UI elements.
 
+    \value [since 6.9] NoTitleBarBackgroundHint Requests that the window's title bar is drawn
+           without a background color. This flag is useful in combination with
+           Qt::ExpandedClientAreaHint, to give the perception that the window's
+           client area seamlessly blends with the titlebar area and controls.
+
     \value WindowType_Mask  A mask for extracting the window type
                             part of the window flags.
 

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -744,6 +744,9 @@ void QCocoaWindow::setWindowFlags(Qt::WindowFlags flags)
     bool ignoreMouse = flags & Qt::WindowTransparentForInput;
     if (m_view.window.ignoresMouseEvents != ignoreMouse)
         m_view.window.ignoresMouseEvents = ignoreMouse;
+
+    m_view.window.titlebarAppearsTransparent = (flags & Qt::NoTitleBarBackgroundHint)
+        || (m_view.window.styleMask & QT_IGNORE_DEPRECATIONS(NSWindowStyleMaskTexturedBackground));
 }
 
 // ----------------------- Window state -----------------------
@@ -2034,7 +2037,6 @@ void QCocoaWindow::applyContentBorderThickness(NSWindow *window)
     if (!m_drawContentBorderGradient) {
         window.styleMask = window.styleMask & ~NSWindowStyleMaskTexturedBackground;
         [window.contentView.superview setNeedsDisplay:YES];
-        window.titlebarAppearsTransparent = NO;
         return;
     }
 
@@ -2059,7 +2061,6 @@ void QCocoaWindow::applyContentBorderThickness(NSWindow *window)
     int effectiveBottomContentBorderThickness = 0;
 
     [window setStyleMask:[window styleMask] | NSWindowStyleMaskTexturedBackground];
-    window.titlebarAppearsTransparent = YES;
 
     // Setting titlebarAppearsTransparent to YES means that the border thickness has to account
     // for the title bar height as well, otherwise sheets will not be presented at the correct

tests/manual/windowflags/controls.cpp

@@ -33,6 +33,7 @@ HintControl::HintControl(QWidget *parent)
     , transparentForInputCheckBox(new QCheckBox(tr("Transparent for input")))
     , noDropShadowCheckBox(new QCheckBox(tr("No drop shadow")))
     , expandedClientAreaCheckBox(new QCheckBox(tr("Expanded client area")))
+    , noTitleBarBackgroundCheckBox(new QCheckBox(tr("No titlebar background")))
 {
     connect(msWindowsFixedSizeDialogCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
     connect(x11BypassWindowManagerCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
@@ -51,6 +52,7 @@ HintControl::HintControl(QWidget *parent)
     connect(transparentForInputCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
     connect(noDropShadowCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
     connect(expandedClientAreaCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
+    connect(noTitleBarBackgroundCheckBox, SIGNAL(clicked()), this, SLOT(slotCheckBoxChanged()));
 
     auto *layout = new QHBoxLayout(this);
     layout->setSpacing(0);
@@ -70,6 +72,7 @@ HintControl::HintControl(QWidget *parent)
     basicHintsLayout->addWidget(msWindowsFixedSizeDialogCheckBox);
     basicHintsLayout->addWidget(x11BypassWindowManagerCheckBox);
     basicHintsLayout->addWidget(expandedClientAreaCheckBox);
+    basicHintsLayout->addWidget(noTitleBarBackgroundCheckBox);
     layout->addLayout(basicHintsLayout);
 
     customizeWindowGroup->setCheckable(true);
@@ -128,6 +131,8 @@ Qt::WindowFlags HintControl::hints() const
         flags |= Qt::NoDropShadowWindowHint;
     if (expandedClientAreaCheckBox->isChecked())
         flags |= Qt::ExpandedClientAreaHint;
+    if (noTitleBarBackgroundCheckBox->isChecked())
+        flags |= Qt::NoTitleBarBackgroundHint;
     return flags;
 }
 
@@ -150,6 +155,7 @@ void HintControl::setHints(Qt::WindowFlags flags)
     transparentForInputCheckBox->setChecked(flags & Qt::WindowTransparentForInput);
     noDropShadowCheckBox->setChecked(flags & Qt::NoDropShadowWindowHint);
     expandedClientAreaCheckBox->setChecked(flags & Qt::ExpandedClientAreaHint);
+    noTitleBarBackgroundCheckBox->setChecked(flags & Qt::NoTitleBarBackgroundHint);
 }
 
 void HintControl::slotCheckBoxChanged()

tests/manual/windowflags/controls.h

@@ -48,6 +48,7 @@ private:
     QCheckBox *transparentForInputCheckBox;
     QCheckBox *noDropShadowCheckBox;
     QCheckBox *expandedClientAreaCheckBox;
+    QCheckBox *noTitleBarBackgroundCheckBox;
 };
 
 // Control for the Qt::WindowState enum, optional with a "visible" QCheckbox