introduce wxBG_STYLE_{ERASE,PAINT} and implement their documented semantics in wxGTK

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61084 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/window.h

@@ -1418,12 +1418,12 @@ public:
 
     /**
         Returns the background style of the window.
-        The background style can be one of the wxBackgroundStyle.
 
         @see SetBackgroundColour(), GetForegroundColour(),
              SetBackgroundStyle(), SetTransparent()
     */
     virtual wxBackgroundStyle GetBackgroundStyle() const;
+
     /**
         Returns the character height for this window.
     */
@@ -1583,8 +1583,28 @@ public:
     virtual bool SetBackgroundColour(const wxColour& colour);
 
     /**
-        Sets the background style of the window. see GetBackgroundStyle() for
-        the description of the possible style values.
+        Sets the background style of the window.
+
+        The default background style is wxBG_STYLE_ERASE which indicates that
+        the window background may be erased in EVT_ERASE_BACKGROUND handler.
+        This is a safe compatibility default however you may want to change it
+        to wxBG_STYLE_SYSTEM if you don't define any erase background event
+        handlers at all to avoid unnecessary generation of erase background
+        events and always let system erase the background. And you should
+        change the background style to wxBG_STYLE_PAINT if you define an
+        EVT_PAINT handler which completely overwrites the window background as
+        in this case erasing it previously, either in EVT_ERASE_BACKGROUND
+        handler or in the system default handler, would result in flicker as
+        the background pixels will be repainted twice every time the window is
+        redrawn. Do ensure that the background is entirely erased by your
+        EVT_PAINT handler in this case however as otherwise garbage may be left
+        on screen.
+
+        Notice that in previous versions of wxWidgets a common way to work
+        around the above mentioned flickering problem was to define an empty
+        EVT_ERASE_BACKGROUND handler. Setting background style to
+        wxBG_STYLE_PAINT is a simpler and more efficient solution to the same
+        problem.
 
         @see SetBackgroundColour(), GetForegroundColour(),
              SetTransparent()