Fix default size_range() calculation (issue #392, STR 3352)

FL/Fl_Group.H

@@ -163,9 +163,12 @@ public:
     See the \ref resize chapter for more examples and detailed explanation.
 
     \note The resizable() widget of a window can also affect the window's
-      resizing behavior if Fl_Window::size_range() is not called.
+      resizing behavior if Fl_Window::size_range() is not called. Please
+      see Fl_Window::default_size_range() for more information on how the
+      default size range is calculated.
 
     \see Fl_Window::size_range()
+    \see Fl_Window::default_size_range()
   */
   void resizable(Fl_Widget* o) {resizable_ = o;}
   /**

FL/Fl_Window.H

@@ -1,7 +1,7 @@
 //
 // Window header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2016 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -74,10 +74,12 @@ private:
 
   const char* iconlabel_;
   char* xclass_;
-  // size_range stuff:
-  int minw, minh, maxw, maxh;
-  int dw, dh, aspect;
-  uchar size_range_set;
+
+  // private size_range stuff:
+  int minw_, minh_, maxw_, maxh_;
+  int dw_, dh_, aspect_;
+  uchar size_range_set_;        // true (1) if size_range() has been set or calculated
+
   // cursor stuff
   Fl_Cursor cursor_default;
 
@@ -119,6 +121,9 @@ protected:
 
   void free_icons();
 
+  void default_size_range();  // calculate size_range() if not set explicitly
+  int is_resizable();         // calculate size_range() and return whether this is resizable
+
 public:
 
   /**
@@ -321,6 +326,7 @@ public:
     \deprecated please use force_position(0) instead
   */
   void free_position()  {clear_flag(FORCE_POSITION);}
+
   void size_range(int minw, int minh, int maxw=0, int maxh=0, int dw=0, int dh=0, int aspect=0);
 
   /** See void Fl_Window::label(const char*)   */