Qt: Styling QTabWidget

As I see it, you have two options, depending on what you want to do.

To fill the background with color:

Take advantage of the transparency of that portion of the tab widget.

QWidget *bg = new QWidget( parent );
bg->setAutoFillBackground( true );
QPalette bg_palette = bg->palette();
bg_palette.setColor( QPalette::Window, QColor( "orange" ) );
bg->setPalette( bg_palette );

QHBoxLayout layout = new QHBoxLayout();
layout->setMargin( 0, 0, 0, 0 );
layout->setSpacing( 0 );
bg->setLayout( layout );

QTabWidget *tab_widget = new QTabWidget();
// set up tab_widget however you want...
layout->addWidget( tab_widget );

This makes the bg widget all orange, but since most of the tab widget will draw over the bg widget, you'll only see the orange where the tab widget doesn't draw.

To make the tabs expand:

I thought this would be easier than it is, but you basically have to subclass QTabWidget in order to get access to the tab bar widget it uses.

class ExpandableTabWidget : public QTabWidget
{
    Q_OBJECT;
    Q_PROPERTY( bool expanding_tabs READ Expanding WRITE SetExpanding );

public:
    ExpandableTabWidget( QWidget *parent = NULL ) : QTabWidget( parent )
    {
    }

    bool Expanding() const
    {
        return tabBar()->expanding();
    }

    void SetTabsExpanding( bool expanding = true )
    {
        tabBar()->setExpanding( expanding );
    }
};

You would then either need to make your ExpandableTabWidget class into a plugin and use it in designer, or you could promote your tab widget to be of type ExpandableTabWidget and set the expanding value in code. If you choose to do the promotion, you won't see the results you want in designer, but you will when you run your program.

Both expanding tabs and coloring the background can be accomplished using style sheets.

For expanding tabs, a style sheet can be applied to the tabs which sets their width to a fraction of the total width of the QTabWidget. Since the style sheet will need to be updated upon resize, it is applied using an event filter. See first example code below.

Although the background of the tab bar can be set, the tab bar doesn't fill the entire space above the tab pane. It is the container (or parent widget) which is showing through. To control the coloring of that area, put the QTabWidget in a QWidget and set the style sheet on the container. See second example code below.

Expanding tabs:

#include <QtGui>

// Sets the style sheet of the QTabWidget to expand the tabs.
static void expandingTabsStyleSheet(QTabWidget *tw)
{
    tw->setStyleSheet(QString("QTabBar::tab { width: %1px; } ")
                      .arg(tw->size().width()/tw->count()));
}

// On resize events, reapply the expanding tabs style sheet
class ResizeFilter : public QObject
{
    QTabWidget *target;
public:
    ResizeFilter(QTabWidget *target) : QObject(target), target(target) {}

    bool eventFilter(QObject *object, QEvent *event)
    {
        if (event->type() == QEvent::Resize)
            expandingTabsStyleSheet(target);
        return false;
    }
};


int main(int argc, char * argv[])
{
  QApplication app(argc, argv);

  QTabWidget *tw = new QTabWidget;
  tw->installEventFilter(new ResizeFilter(tw));
  tw->addTab(new QWidget, "Tab1");
  tw->addTab(new QWidget, "Tab2");
  tw->addTab(new QWidget, "Tab3");

  tw->show();

  return app.exec();
}

Background beside tabs:

#include <QtGui>

int main(int argc, char * argv[])
{
  QApplication app(argc, argv);

  QWidget *container = new QWidget;
  container->setStyleSheet("background: qlineargradient( x1: 0, y1: 0, x2: 1, y2
: 0, stop: 0 black, stop: 1 blue);");

  QHBoxLayout *layout = new QHBoxLayout(container);
  layout->setContentsMargins(0, 0, 0, 0);

  QTabWidget *tw = new QTabWidget(container);
  layout->addWidget(tw);
  tw->setStyleSheet(
      "QTabBar::tab { background: gray; color: white; padding: 10px; } "
      "QTabBar::tab:selected { background: lightgray; } "
      "QTabWidget::pane { border: 0; } "
      "QWidget { background: lightgray; } ");
  tw->addTab(new QWidget, "Tab1");
  tw->addTab(new QWidget, "Tab2");
  tw->addTab(new QWidget, "Tab3");

  container->show();

  return app.exec();
}

On qt 4.5 there is a new property that control the tab widget rendering, called documentMode. Just call tabWidget->setDocumentMode(true) and set the background color of the QTabBar using the stylesheets.

From Qt Documentation:

This property holds whether or not the tab widget is rendered in a mode suitable for document pages. This is the same as document mode on Mac OS X.

When this property is set the tab widget frame is not rendered. This mode is useful for showing document-type pages where the page covers most of the tab widget area.