[81_4] Implement multi-line display for numerous tab pages

## What
See in
[https://github.com/XmacsLabs/mogan/pull/1961](https://github.com/XmacsLabs/mogan/pull/1961)
## How to test your changes?
Create numerous tab pages using `Ctrl+N`, and then maximize/minimize the
window to view changes.

src/Plugins/Qt/QTMTabPage.cpp

@@ -11,18 +11,21 @@
 
 #include "QTMTabPage.hpp"
 
+// The minimum width of a single tab page (in pixels).
+#define MIN_TAB_PAGE_WIDTH 150
+
 /******************************************************************************
  * QTMTabPage
  ******************************************************************************/
 
 QTMTabPage::QTMTabPage (url p_url, QAction* p_title, QAction* p_closeBtn,
                         bool p_isActive)
-    : bufferUrl (p_url) {
+    : m_bufferUrl (p_url) {
   p_title->setCheckable (p_isActive);
   p_title->setChecked (p_isActive);
-  closeBtn= new QToolButton (this);
-  closeBtn->setDefaultAction (p_closeBtn);
-  closeBtn->setFixedSize (20, 20); // position will be updated in resizeEvent
+  m_closeBtn= new QToolButton (this);
+  m_closeBtn->setDefaultAction (p_closeBtn);
+  m_closeBtn->setFixedSize (20, 20); // position will be updated in resizeEvent
 
   setStyleSheet (
       "QToolButton{ padding: 0 26px; }"); // reserve space for closeBtn
@@ -31,10 +34,109 @@ QTMTabPage::QTMTabPage (url p_url, QAction* p_title, QAction* p_closeBtn,
 
 void
 QTMTabPage::resizeEvent (QResizeEvent* e) {
-  int w= closeBtn->width ();
-  int h= closeBtn->height ();
+  int w= m_closeBtn->width ();
+  int h= m_closeBtn->height ();
   int x= e->size ().width () - w - 8;
   int y= e->size ().height () / 2 - h / 2;
 
-  closeBtn->setGeometry (x, y, w, h);
+  m_closeBtn->setGeometry (x, y, w, h);
+}
+
+/******************************************************************************
+ * QTMTabPageContainer
+ ******************************************************************************/
+
+QTMTabPageContainer::QTMTabPageContainer (QWidget* p_parent)
+    : QWidget (p_parent) {
+  setSizePolicy (QSizePolicy::Expanding, QSizePolicy::Preferred);
+}
+
+QTMTabPageContainer::~QTMTabPageContainer () { removeAllTabPages (); }
+
+void
+QTMTabPageContainer::replaceTabPages (QList<QAction*>* p_src) {
+  removeAllTabPages ();    // remove  old tabs
+  extractTabPages (p_src); // extract new tabs
+
+  const int windowWidth= this->width ();
+  int       rowCount   = 0;
+  int       accumWidth = 0;
+  int       accumTab   = 0;
+
+  for (int i= 0; i < m_tabPageList.size (); ++i) {
+    QTMTabPage* tab= m_tabPageList[i];
+    tab->setParent (this);
+
+    QSize tabSize = tab->minimumSizeHint ();
+    int   tabWidth= max (MIN_TAB_PAGE_WIDTH, tabSize.width ());
+    if (accumWidth + tabWidth >= windowWidth) {
+      rowCount+= 1;
+      accumWidth= 0;
+      accumTab  = 0;
+    }
+    tab->setGeometry (accumWidth - accumTab, rowCount * m_rowHeight - rowCount,
+                      tabWidth, m_rowHeight);
+    accumWidth+= tabWidth;
+    accumTab+= 1;
+  }
+
+  adjustHeight (rowCount);
+}
+
+void
+QTMTabPageContainer::removeAllTabPages () {
+  for (int i= 0; i < m_tabPageList.size (); ++i) {
+    delete m_tabPageList[i];
+  }
+  m_tabPageList.clear ();
+}
+
+void
+QTMTabPageContainer::extractTabPages (QList<QAction*>* p_src) {
+  if (!p_src) return;
+  for (int i= 0; i < p_src->size (); ++i) {
+    // see the definition of QTMTabPageAction why we're using it
+    QTMTabPageAction* carrier= qobject_cast<QTMTabPageAction*> ((*p_src)[i]);
+    ASSERT (carrier, "QTMTabPageAction expected")
+
+    QTMTabPage* tab= qobject_cast<QTMTabPage*> (carrier->m_widget);
+    if (tab) m_tabPageList.append (tab);
+
+    delete carrier; // we don't need it anymore
+  }
+}
+
+void
+QTMTabPageContainer::adjustHeight (int p_rowCount) {
+  int h= m_rowHeight * (p_rowCount + 1);
+  // parentWidget's resizeEvent() will resize me
+  parentWidget ()->setFixedHeight (h - p_rowCount + 1);
+}
+
+/******************************************************************************
+ * QTMTabPageBar
+ ******************************************************************************/
+
+QTMTabPageBar::QTMTabPageBar (const QString& p_title, QWidget* p_parent)
+    : QToolBar (p_title, p_parent) {
+  m_container= new QTMTabPageContainer (this);
+}
+
+void
+QTMTabPageBar::replaceTabPages (QList<QAction*>* p_src) {
+  setUpdatesEnabled (false);
+  bool visible= this->isVisible ();
+  if (visible) hide (); // TRICK: to avoid flicker of the dest widget
+
+  m_container->replaceTabPages (p_src);
+
+  if (visible) show (); // TRICK: see above
+  setUpdatesEnabled (true);
+}
+
+void
+QTMTabPageBar::resizeEvent (QResizeEvent* e) {
+  QSize size= e->size ();
+  // Reserve 7px space on the left for the handle of QToolbar
+  m_container->setGeometry (7, 0, size.width (), size.height ());
 }

src/Plugins/Qt/QTMTabPage.hpp

@@ -21,10 +21,10 @@
  */
 class QTMTabPage : public QToolButton {
   Q_OBJECT
-  QToolButton* closeBtn;
+  QToolButton* m_closeBtn;
 
 public:
-  const url bufferUrl;
+  const url m_bufferUrl;
 
 public:
   explicit QTMTabPage (url p_url, QAction* p_title, QAction* p_closeBtn,
@@ -34,15 +34,61 @@ class QTMTabPage : public QToolButton {
   virtual void resizeEvent (QResizeEvent* e) override;
 };
 
+/*! QTMTabPageAction is used as a carrier of QTMTabPage widget.
+Why:
+If we use the QWidgetAction, once we call setDefaultWidget,
+we can't take out the defaultWidget from QWidgetAction, because
+when we delete the QWidgetAction, the defaultWidget will also be
+deleted. You can see this behavior in the source code of QWidgetAction.
+ */
+class QTMTabPageAction : public QAction {
+  Q_OBJECT
+public:
+  explicit QTMTabPageAction (QWidget* p_widget) : m_widget (p_widget) {}
+  QWidget* const m_widget;
+};
+
+/*! QTMTabPageContainer is used to build the container for QTMTabPage.
+In order to:
+1. Support multi-line display for numerous tab pages;
+2. Support drag-and-drop to sort tab page
+ */
+class QTMTabPageContainer : public QWidget {
+  QList<QTMTabPage*> m_tabPageList;
+  int                m_rowHeight= 0;
+
+public:
+  explicit QTMTabPageContainer (QWidget* p_parent);
+  ~QTMTabPageContainer ();
+
+  inline void setRowHeight (int p_height) { m_rowHeight= p_height; }
+  void        replaceTabPages (QList<QAction*>* p_src);
+
+protected:
+  void removeAllTabPages ();
+  void extractTabPages (QList<QAction*>* p_src);
+  void adjustHeight (int p_rowCount);
+};
+
 /*! QTMTabPageBar is used to wrap the QTMTabPageContainer.
 In order to:
 1. Add this to the QMainWindow as QToolBar, just like the main icon toolbar;
 2. Support dragging and docking like the main icon toolbar.
  */
 class QTMTabPageBar : public QToolBar {
+  QTMTabPageContainer* m_container;
+
 public:
-  explicit QTMTabPageBar (const QString& p_title, QWidget* p_parent)
-      : QToolBar (p_title, p_parent) {}
+  explicit QTMTabPageBar (const QString& p_title, QWidget* p_parent);
+
+  inline void setRowHeight (int p_height) {
+    m_container->setRowHeight (p_height);
+  }
+
+  void replaceTabPages (QList<QAction*>* p_src);
+
+protected:
+  virtual void resizeEvent (QResizeEvent* e) override;
 };
 
 #endif // QTMTABPAGE_HPP

src/Plugins/Qt/qt_tm_widget.cpp

@@ -248,13 +248,13 @@ qt_tm_widget_rep::qt_tm_widget_rep (int mask, command _quit)
   mainToolBar->setFixedHeight (toolbarHeight + 8 * retina_icons);
   modeToolBar->setFixedHeight (toolbarHeight + 4 * retina_icons);
   focusToolBar->setFixedHeight (toolbarHeight);
-  tabToolBar->setFixedHeight (toolbarHeight + 4 * retina_icons);
+  tabToolBar->setRowHeight (toolbarHeight + 4 * retina_icons);
 #else
   int toolbarHeight= 30;
   mainToolBar->setFixedHeight (toolbarHeight + 8);
   modeToolBar->setFixedHeight (toolbarHeight + 4);
   focusToolBar->setFixedHeight (toolbarHeight);
-  tabToolBar->setFixedHeight (toolbarHeight + 4);
+  tabToolBar->setRowHeight (toolbarHeight + 4);
 #endif
   if (tm_style_sheet != "") {
     double scale= retina_scale;
@@ -264,7 +264,7 @@ qt_tm_widget_rep::qt_tm_widget_rep (int mask, command _quit)
     mainToolBar->setFixedHeight (h1);
     modeToolBar->setFixedHeight (h2);
     focusToolBar->setFixedHeight (h3);
-    tabToolBar->setFixedHeight (h2);
+    tabToolBar->setRowHeight (h2);
   }
 
   QWidget* cw= new QWidget ();
@@ -897,7 +897,7 @@ qt_tm_widget_rep::write (slot s, blackbox index, widget w) {
       tab_bar_widget       = concrete (w);
       QList<QAction*>* list= tab_bar_widget->get_qactionlist ();
       if (list) {
-        replaceButtons (tabToolBar, list);
+        tabToolBar->replaceTabPages (list);
         update_visibility ();
       }
     }

src/Plugins/Qt/qt_ui_element.cpp

@@ -525,11 +525,13 @@ qt_ui_element_rep::as_qaction () {
   case tab_page_widget: {
     typedef quartet<url, widget, widget, bool> T;
     T                                          x= open_box<T> (load);
-    QTMTabPage*    tab= new QTMTabPage (x.x1, concrete (x.x2)->as_qaction (),
-                                        concrete (x.x3)->as_qaction (), x.x4);
-    QWidgetAction* a  = new QWidgetAction (nullptr);
-    a->setDefaultWidget (tab);
-    act= a;
+    QTMTabPage* tab= new QTMTabPage (x.x1, concrete (x.x2)->as_qaction (),
+                                     concrete (x.x3)->as_qaction (), x.x4);
+    // use QTMTabPageAction to wrap the QTMTabPage(QWidget type)
+    // then we take out the QTMTabPage and add it to the QTMTabPageBar
+    // see the definition of QTMTabPageAction why we're using it
+    QTMTabPageAction* a= new QTMTabPageAction (tab);
+    act                = a;
   } break;
 
   default: