Windows: improve changing scale values of display while app runs.

src/Fl_Screen_Driver.H

@@ -247,6 +247,7 @@ public:
   virtual void set_spot(int font, int size, int X, int Y, int W, int H, Fl_Window *win);
   virtual void reset_spot();
   virtual void set_status(int X, int Y, int W, int H);
+  virtual float base_scale(int numscreen);
 };
 
 #endif // !FL_SCREEN_DRIVER_H

src/Fl_Screen_Driver.cxx

@@ -505,7 +505,7 @@ int Fl_Screen_Driver::scale_handler(int event)
       }
       top = Fl::next_window(top);
     }
-    static float initial_scale = screen_dr->scale(screen);
+    float initial_scale = screen_dr->base_scale(screen);
 #if defined(TEST_SCALING)
     // test scaling factors: lots of values from 0.3 to 8.0
     static float scaling_values[] = {
@@ -791,6 +791,13 @@ size_t Fl_Screen_Driver::convert_crlf(char *s, size_t len) {
   return len;
 }
 
+
+float Fl_Screen_Driver::base_scale(int numscreen) {
+  static float base = scale(numscreen);
+  return base;
+}
+
+
 /**
  \}
  \endcond

src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.H

@@ -97,6 +97,7 @@ public:
   // these two are implemented in Fl_get_key_win32.cxx
   int event_key(int) FL_OVERRIDE;
   int get_key(int) FL_OVERRIDE;
+  float base_scale(int numscreen) FL_OVERRIDE;
 };
 
 

src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx

@@ -483,3 +483,8 @@ int Fl_WinAPI_Screen_Driver::screen_num_unscaled(int x, int y)
   }
   return screen;
 }
+
+
+float Fl_WinAPI_Screen_Driver::base_scale(int numscreen) {
+  return dpi[numscreen][0] / 96.;
+}