Support re-ordering tabs with mouse

Just set KDDockWidgets::Config::Flag_AllowReorderTabs before creating
the dock widgets.

Fixes #20

Changelog

@@ -0,0 +1,6 @@
+* v1.0 (November 4th, 2019)
+  - Initial Release
+
+* v1.1 (, 2020)
+  - Fix dock widgets not filling their complete available space
+  - Allow tab re-ordering with mouse, via KDDockWidgets::Config::Flag_AllowReorderTabs.

examples/dockwidgets/main.cpp

@@ -49,6 +49,9 @@ int main(int argc, char **argv)
     QCommandLineOption customStyle("p", QCoreApplication::translate("main", "Shows how to style framework internals via FrameworkWidgetFactory"));
     parser.addOption(customStyle);
 
+    QCommandLineOption reorderTabsOption("r", QCoreApplication::translate("main", "Support re-ordering tabs with mouse"));
+    parser.addOption(reorderTabsOption);
+
     QCommandLineOption noTitleBars("t", QCoreApplication::translate("main", "Never show titlebars"));
     parser.addOption(noTitleBars);
 
@@ -72,8 +75,14 @@ int main(int argc, char **argv)
                                            : MainWindowOption_HasCentralFrame;
 #endif
 
+    auto flags = KDDockWidgets::Config::self().flags();
     if (parser.isSet(noTitleBars))
-        KDDockWidgets::Config::self().setFlags(KDDockWidgets::Config::Flags() | KDDockWidgets::Config::Flag_HideTitleBarWhenTabsVisible | KDDockWidgets::Config::Flag_AlwaysShowTabs);
+        flags |= KDDockWidgets::Config::Flag_HideTitleBarWhenTabsVisible | KDDockWidgets::Config::Flag_AlwaysShowTabs;
+
+    if (parser.isSet(reorderTabsOption))
+        flags |= KDDockWidgets::Config::Flag_AllowReorderTabs;
+
+    KDDockWidgets::Config::self().setFlags(flags);
 
     MyMainWindow mainWindow(options);
     mainWindow.resize(1200, 1200);

src/Config.h

@@ -64,7 +64,8 @@ class DOCKS_EXPORT Config
         Flag_NativeTitleBar = 1, ///> Enables the Native OS title bar on OSes that support it (Windows 10, macOS), ignored otherwise. This is mutually exclusive with Flag_AeroSnap
         Flag_AeroSnapWithClientDecos = 2, ///> Enables AeroSnap even if we're not using the native title bar. Only supported on Windows 10.
         Flag_HideTitleBarWhenTabsVisible = 8, ///> Hides the title bar if there's tabs visible. The empty space in the tab bar becomes draggable.
-        Flag_AlwaysShowTabs = 16, ///> Always show tabs, even if there's only one
+        Flag_AlwaysShowTabs = 16, ///> Always show tabs, even if there's only one,
+        Flag_AllowReorderTabs = 32, /// Allows user to re-order tabs by dragging them
         Flag_Default = Flag_AeroSnapWithClientDecos ///> The defaults
     };
     Q_DECLARE_FLAGS(Flags, Flag)

src/private/DragController.cpp

@@ -155,17 +155,17 @@ bool StatePreDrag::handleMouseMove(QPoint globalPos)
 {
     if (q->m_draggable->dragCanStart(q->m_pressPos, globalPos)) {
         Q_EMIT q->manhattanLengthMove();
+        return true;
     }
-    return true;
+    return false;
 }
 
 bool StatePreDrag::handleMouseButtonRelease(QPoint)
 {
     Q_EMIT q->dragCanceled();
-    return true;
+    return false;
 }
 
-
 StateDragging::StateDragging(DragController *parent)
     : StateBase(parent)
 {

src/private/widgets/TabBarWidget.cpp

@@ -27,15 +27,18 @@
  */
 
 #include "TabBarWidget_p.h"
+#include "Config.h"
 
 #include <QMouseEvent>
+#include <QApplication>
 
 using namespace KDDockWidgets;
 
 TabBarWidget::TabBarWidget(TabWidget *parent)
     : QTabBar(parent->asWidget())
     , TabBar(this, parent)
 {
+    setMovable(Config::self().flags() & Config::Flag_AllowReorderTabs);
 }
 
 int TabBarWidget::numDockWidgets() const
@@ -53,3 +56,36 @@ void TabBarWidget::mousePressEvent(QMouseEvent *e)
     onMousePress(e->pos());
     QTabBar::mousePressEvent(e);
 }
+
+bool TabBarWidget::dragCanStart(QPoint pressPos, QPoint pos) const
+{
+    // Here we allow the user to re-order tabs instead of dragging them off.
+    // To do that we just return false here, and QTabBar will handle the mouse event, assuming QTabBar::isMovable.
+
+    const bool defaultResult = Draggable::dragCanStart(pressPos, pos);
+
+    if (!defaultResult || !isMovable()) {
+        // Nothing more to do. If the drag wouldn't start anyway, return false.
+        // And if the tabs aren't movable, just return the default result, which just considers
+        // QApplication::startDragDistances
+        return defaultResult;
+    }
+
+    const int index = tabAt(mapFromGlobal(pos));
+    if (index == -1)
+        return defaultResult;
+
+    const int deltaX = qAbs(pos.x() - pressPos.x());
+    const int deltaY = qAbs(pos.y() - pressPos.y());
+
+    if (deltaY > 5 * QApplication::startDragDistance()) {
+        // Moving up or down too much results in a detach. No tab re-ordering allowed.
+        return true;
+    } else if (deltaY > QApplication::startDragDistance() && deltaX < QApplication::startDragDistance()) {
+        // Moved a bit up or down, but not left/right, then detach too.
+        // Only if it's going considerably left/right we allow to re-order tabs.
+        return true;
+    }
+
+    return false;
+}

src/private/widgets/TabBarWidget_p.h

@@ -50,6 +50,7 @@ class DOCKS_EXPORT TabBarWidget : public QTabBar, public TabBar
     int numDockWidgets() const override;
     int tabAt(QPoint localPos) const override;
 protected:
+    bool dragCanStart(QPoint pressPos, QPoint pos) const override;
     void mousePressEvent(QMouseEvent *) override;
 
 };