Fix the maximization of sub windows (LMMS#7530)

## Fix rendering of maximized sub windows

### Adjustments in paintEvent
In `SubWindow::paintEvent` don't paint anything if the sub window is
maximized . Otherwise some gradients are visible behind the maximized
child content.

### Adjustments in adjustTitleBar
In `SubWindow::adjustTitleBar` hide the title label and the buttons if the
sub window is maximized. Always show the title and close button if not
maximized. This is needed to reset the state correctly after
maximization.

Remove some calls to `isMaximized` where we already know that the sub
window is not maximized, i.e. where these calls would always return
`false`.

One adjustment would have resulted in a call to `setHidden(false)`. This
was changed to `setVisible(true)` to get rid of the double negation.

The other `false` would have gotten in an or statement and thus could be
removed completely.

### Add method addTitleButton
Add the helper method `SubWindow::addTitleButton` to reduce code
repetition in the constructor.

### Other changes
Remove a dependency on the `MdiArea` when checking if the sub window is
the active one. Query its own window state to find out if it is active.

When calling `setWindowFlags` in the constructor only adjust the existing
flags with the changes that we actually want to do. It was ensured that
all other flags that have been set before still apply with this change.

include/SubWindow.h

@@ -78,6 +78,8 @@ class LMMS_EXPORT SubWindow : public QMdiSubWindow
 	void paintEvent( QPaintEvent * pe ) override;
 	void changeEvent( QEvent * event ) override;
 
+	QPushButton* addTitleButton(const std::string& iconName, const QString& toolTip);
+
 signals:
 	void focusLost();
 

src/gui/SubWindow.cpp

@@ -58,28 +58,13 @@ SubWindow::SubWindow(QWidget *parent, Qt::WindowFlags windowFlags) :
 	m_borderColor = Qt::black;
 
 	// close, maximize and restore (after maximizing) buttons
-	m_closeBtn = new QPushButton( embed::getIconPixmap( "close" ), QString(), this );
-	m_closeBtn->resize( m_buttonSize );
-	m_closeBtn->setFocusPolicy( Qt::NoFocus );
-	m_closeBtn->setCursor( Qt::ArrowCursor );
-	m_closeBtn->setAttribute( Qt::WA_NoMousePropagation );
-	m_closeBtn->setToolTip( tr( "Close" ) );
+	m_closeBtn = addTitleButton("close", tr("Close"));
 	connect( m_closeBtn, SIGNAL(clicked(bool)), this, SLOT(close()));
 
-	m_maximizeBtn = new QPushButton( embed::getIconPixmap( "maximize" ), QString(), this );
-	m_maximizeBtn->resize( m_buttonSize );
-	m_maximizeBtn->setFocusPolicy( Qt::NoFocus );
-	m_maximizeBtn->setCursor( Qt::ArrowCursor );
-	m_maximizeBtn->setAttribute( Qt::WA_NoMousePropagation );
-	m_maximizeBtn->setToolTip( tr( "Maximize" ) );
+	m_maximizeBtn = addTitleButton("maximize", tr("Maximize"));
 	connect( m_maximizeBtn, SIGNAL(clicked(bool)), this, SLOT(showMaximized()));
 
-	m_restoreBtn = new QPushButton( embed::getIconPixmap( "restore" ), QString(), this );
-	m_restoreBtn->resize( m_buttonSize );
-	m_restoreBtn->setFocusPolicy( Qt::NoFocus );
-	m_restoreBtn->setCursor( Qt::ArrowCursor );
-	m_restoreBtn->setAttribute( Qt::WA_NoMousePropagation );
-	m_restoreBtn->setToolTip( tr( "Restore" ) );
+	m_restoreBtn = addTitleButton("restore", tr("Restore"));
 	connect( m_restoreBtn, SIGNAL(clicked(bool)), this, SLOT(showNormal()));
 
 	// QLabel for the window title and the shadow effect
@@ -93,10 +78,9 @@ SubWindow::SubWindow(QWidget *parent, Qt::WindowFlags windowFlags) :
 	m_windowTitle->setAttribute( Qt::WA_TransparentForMouseEvents, true );
 	m_windowTitle->setGraphicsEffect( m_shadow );
 
-	// disable the minimize button
-	setWindowFlags( Qt::SubWindow | Qt::WindowMaximizeButtonHint |
-		Qt::WindowSystemMenuHint | Qt::WindowCloseButtonHint |
-		Qt::CustomizeWindowHint );
+	// Disable the minimize button and make sure that the custom window hint is set
+	setWindowFlags((this->windowFlags() & ~Qt::WindowMinimizeButtonHint) | Qt::CustomizeWindowHint);
+
 	connect( mdiArea(), SIGNAL(subWindowActivated(QMdiSubWindow*)), this, SLOT(focusChanged(QMdiSubWindow*)));
 }
 
@@ -111,12 +95,14 @@ SubWindow::SubWindow(QWidget *parent, Qt::WindowFlags windowFlags) :
  */
 void SubWindow::paintEvent( QPaintEvent * )
 {
+	// Don't paint any of the other stuff if the sub window is maximized
+	// so that only its child content is painted.
+	if (isMaximized()) { return; }
+
 	QPainter p( this );
 	QRect rect( 0, 0, width(), m_titleBarHeight );
 
-	bool isActive = mdiArea()
-			? mdiArea()->activeSubWindow() == this
-			: false;
+	const bool isActive = windowState() & Qt::WindowActive;
 
 	p.fillRect( rect, isActive ? activeColor() : p.pen().brush() );
 
@@ -295,9 +281,28 @@ void SubWindow::moveEvent( QMoveEvent * event )
  */
 void SubWindow::adjustTitleBar()
 {
+	// Don't show the title or any button if the sub window is maximized. Otherwise they
+	// might show up behind the actual maximized content of the child widget.
+	if (isMaximized())
+	{
+		m_closeBtn->hide();
+		m_maximizeBtn->hide();
+		m_restoreBtn->hide();
+		m_windowTitle->hide();
+
+		return;
+	}
+
+	// The sub window is not maximized, i.e. the title must be shown
+	// as well as some buttons.
+
+	// Title adjustments
+	m_windowTitle->show();
+
 	// button adjustments
 	m_maximizeBtn->hide();
 	m_restoreBtn->hide();
+	m_closeBtn->show();
 
 	const int rightSpace = 3;
 	const int buttonGap = 1;
@@ -322,12 +327,12 @@ void SubWindow::adjustTitleBar()
 		buttonBarWidth = buttonBarWidth + m_buttonSize.width() + buttonGap;
 		m_maximizeBtn->move( middleButtonPos );
 		m_restoreBtn->move( middleButtonPos );
-		m_maximizeBtn->setHidden( isMaximized() );
+		m_maximizeBtn->setVisible(true);
 	}
 
 	// we're keeping the restore button around if we open projects
 	// from older versions that have saved minimized windows
-	m_restoreBtn->setVisible( isMaximized() || isMinimized() );
+	m_restoreBtn->setVisible(isMinimized());
 	if( isMinimized() )
 	{
 		m_restoreBtn->move( m_maximizeBtn->isHidden() ?  middleButtonPos : leftButtonPos );
@@ -403,5 +408,17 @@ void SubWindow::resizeEvent( QResizeEvent * event )
 	}
 }
 
+QPushButton* SubWindow::addTitleButton(const std::string& iconName, const QString& toolTip)
+{
+	auto button = new QPushButton(embed::getIconPixmap(iconName), QString(), this);
+	button->resize(m_buttonSize);
+	button->setFocusPolicy(Qt::NoFocus);
+	button->setCursor(Qt::ArrowCursor);
+	button->setAttribute(Qt::WA_NoMousePropagation);
+	button->setToolTip(toolTip);
+
+	return button;
+}
+
 
-} // namespace lmms::gui
+} // namespace lmms::gui