[plot] Docked widgets always shown at bottom when instanstiated
We seem to have lost the functionality of showing docked widgets to the right of the plot when the plot was much wider than tall.
diff --git a/silx/gui/plot/PlotWindow.py b/silx/gui/plot/PlotWindow.py
index a39430ebe..ceafa23de 100644
--- a/silx/gui/plot/PlotWindow.py
+++ b/silx/gui/plot/PlotWindow.py
@@ -122,7 +122,7 @@ class PlotWindow(PlotWidget):
         self._curvesROIDockWidget = None
         self._maskToolsDockWidget = None
         self._consoleDockWidget = None
-        self._statsWidget = None
+        self._statsDockWidget = None
 
         # Create color bar, hidden by default for backward compatibility
         self._colorbar = ColorBarWidget(parent=self, plot=self)
@@ -485,6 +485,22 @@ class PlotWindow(PlotWidget):
             self.tabifyDockWidget(self._dockWidgets[0],
                                   dock_widget)
 
+    def __handleFirstDockWidgetShow(self, visible):
+        """Handle QDockWidget.visibilityChanged
+
+        It calls :meth:`addTabbedDockWidget` for the `sender` widget.
+        This allows to call `addTabbedDockWidget` lazily.
+
+        It disconnect itself from the signal once done.
+
+        :param bool visible:
+        """
+        if visible:
+            dockWidget = self.sender()
+            dockWidget.visibilityChanged.disconnect(
+                self.__handleFirstDockWidgetShow)
+            self.addTabbedDockWidget(dockWidget)
+
     def getColorBarWidget(self):
         """Returns the embedded :class:`ColorBarWidget` widget.
 
@@ -499,7 +515,8 @@ class PlotWindow(PlotWidget):
         if self._legendsDockWidget is None:
             self._legendsDockWidget = LegendsDockWidget(plot=self)
             self._legendsDockWidget.hide()
-            self.addTabbedDockWidget(self._legendsDockWidget)
+            self._legendsDockWidget.visibilityChanged.connect(
+                self.__handleFirstDockWidgetShow)
         return self._legendsDockWidget
 
     def getCurvesRoiDockWidget(self):
@@ -509,7 +526,8 @@ class PlotWindow(PlotWidget):
             self._curvesROIDockWidget = CurvesROIDockWidget(
                 plot=self, name='Regions Of Interest')
             self._curvesROIDockWidget.hide()
-            self.addTabbedDockWidget(self._curvesROIDockWidget)
+            self._curvesROIDockWidget.visibilityChanged.connect(
+                self.__handleFirstDockWidgetShow)
         return self._curvesROIDockWidget
 
     def getCurvesRoiWidget(self):
@@ -529,7 +547,8 @@ class PlotWindow(PlotWidget):
             self._maskToolsDockWidget = MaskToolsDockWidget(
                 plot=self, name='Mask')
             self._maskToolsDockWidget.hide()
-            self.addTabbedDockWidget(self._maskToolsDockWidget)
+            self._maskToolsDockWidget.visibilityChanged.connect(
+                self.__handleFirstDockWidgetShow)
         return self._maskToolsDockWidget
 
     def getStatsWidget(self):
@@ -537,16 +556,18 @@ class PlotWindow(PlotWidget):
 
         :rtype: BasicStatsWidget
         """
-        if self._statsWidget is None:
-            dockWidget = qt.QDockWidget(parent=self)
-            dockWidget.setWindowTitle("Curves stats")
-            dockWidget.layout().setContentsMargins(0, 0, 0, 0)
-            self._statsWidget = BasicStatsWidget(parent=self, plot=self)
-            self._statsWidget.sigVisibilityChanged.connect(self.getStatsAction().setChecked)
-            dockWidget.setWidget(self._statsWidget)
-            dockWidget.hide()
-            self.addTabbedDockWidget(dockWidget)
-        return self._statsWidget
+        if self._statsDockWidget is None:
+            self._statsDockWidget = qt.QDockWidget()
+            self._statsDockWidget.setWindowTitle("Curves stats")
+            self._statsDockWidget.layout().setContentsMargins(0, 0, 0, 0)
+            statsWidget = BasicStatsWidget(parent=self, plot=self)
+            statsWidget.sigVisibilityChanged.connect(
+                self.getStatsAction().setChecked)
+            self._statsDockWidget.setWidget(statsWidget)
+            self._statsDockWidget.hide()
+            self._statsDockWidget.visibilityChanged.connect(
+                self.__handleFirstDockWidgetShow)
+        return self._statsDockWidget.widget()
 
     # getters for actions
     @property
['silx/gui/plot/PlotWindow.py']