Process env var FLTK_SCALING_FACTOR consistently across platforms.

The procedure to set screen scaling factors becomes:
1) each screen scaling factor is set to 1
2) the OS is queried according to each platform to get screen scaling factor
values
3) The value of FLTK_SCALING_FACTOR, if present, is used to multiply
scaling factors

src/Fl_Screen_Driver.H

@@ -175,8 +175,8 @@ public:
   void rescale_all_windows_from_screen(int screen, float f);
   static void transient_scale_display(float f, int nscreen);
   static int scale_handler(int event);
-  virtual float desktop_scale_factor() {return 1;}
-  float use_startup_scale_factor();
+  virtual void desktop_scale_factor() {}
+  void use_startup_scale_factor();
   enum APP_SCALING_CAPABILITY {
     NO_APP_SCALING = 0, ///< The platform does not support rescaling.
     SYSTEMWIDE_APP_SCALING, ///< The platform supports rescaling with the same factor for all screens.

src/Fl_Screen_Driver.cxx

@@ -434,20 +434,15 @@ int Fl_Screen_Driver::scale_handler(int event)
 
 
 // use the startup time scaling value
-float Fl_Screen_Driver::use_startup_scale_factor()
+void Fl_Screen_Driver::use_startup_scale_factor()
 {
-  float factor;
-  char *p = 0;
+  char *p;
+  desktop_scale_factor();
   if ((p = fl_getenv("FLTK_SCALING_FACTOR"))) {
+    float factor = 1;
     sscanf(p, "%f", &factor);
+    for (int i = 0; i < screen_count(); i++)  scale(i, factor * scale(i));
   }
-  else {
-    factor = desktop_scale_factor();
-  }
-  // checks to prevent potential crash (factor <= 0) or very large factors
-  if (factor < 0.25) factor = 0.25;
-  else if (factor > 10.0) factor = 10.0;
-  return factor;
 }
 
 
@@ -457,10 +452,8 @@ void Fl_Screen_Driver::open_display()
   static bool been_here = false;
   if (!been_here) {
     been_here = true;
-    int scount = screen_count(); // keep here
     if (rescalable()) {
-      float factor = use_startup_scale_factor();
-      if (factor) for (int i = 0; i < scount; i++)  scale(i, factor);
+      use_startup_scale_factor();
       Fl::add_handler(Fl_Screen_Driver::scale_handler);
       int mx, my;
       int ns = Fl::screen_driver()->get_mouse(mx, my);

src/Fl_win32.cxx

@@ -548,8 +548,7 @@ void Fl_WinAPI_Screen_Driver::open_display_platform() {
     const int PROCESS_PER_MONITOR_DPI_AWARE = 2;
     if (fl_SetProcessDpiAwareness) {
       HRESULT hr = fl_SetProcessDpiAwareness(PROCESS_PER_MONITOR_DPI_AWARE);
-      if (hr == S_OK) init_screen_scale_factors();
-      else fl_SetProcessDpiAwareness = NULL;
+      if (hr != S_OK) fl_SetProcessDpiAwareness = NULL;
     }
   }
   OleInitialize(0L);
@@ -558,11 +557,11 @@ void Fl_WinAPI_Screen_Driver::open_display_platform() {
 }
 
 
-void Fl_WinAPI_Screen_Driver::init_screen_scale_factors() {
+void Fl_WinAPI_Screen_Driver::desktop_scale_factor() {
   typedef HRESULT(WINAPI * GetDpiForMonitor_type)(HMONITOR, int, UINT *, UINT *);
   HMODULE hMod = LoadLibrary("Shcore.DLL");
   GetDpiForMonitor_type fl_GetDpiForMonitor = NULL;
-  if (hMod)
+  if (hMod && fl_SetProcessDpiAwareness)
     fl_GetDpiForMonitor = (GetDpiForMonitor_type)GetProcAddress(hMod, "GetDpiForMonitor");
   if (fl_GetDpiForMonitor) {
     for (int ns = 0; ns < screen_count(); ns++) {

src/Fl_x.cxx

@@ -1255,9 +1255,9 @@ static void react_to_screen_reconfiguration() {
     for (int screen = 0; screen < old_count; screen++)
       Fl::screen_driver()->scale(screen, scales[screen]);
   } else {
-    float factor = Fl::screen_driver()->use_startup_scale_factor();
+    Fl::screen_driver()->use_startup_scale_factor();
     for (int screen = 0; screen < Fl::screen_count(); screen++)
-      Fl::screen_driver()->rescale_all_windows_from_screen(screen, factor);
+      Fl::screen_driver()->rescale_all_windows_from_screen(screen, Fl::screen_driver()->scale(screen));
   }
   delete[] scales;
 #endif // USE_XFT

src/drivers/Cocoa/Fl_Cocoa_Screen_Driver.cxx

@@ -78,6 +78,7 @@ static Fl_Text_Editor::Key_Binding extra_bindings[] =  {
 
 Fl_Cocoa_Screen_Driver::Fl_Cocoa_Screen_Driver() {
   text_editor_extra_key_bindings =  extra_bindings;
+  scale_ = 1.;
 }
 
 

src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.H

@@ -42,7 +42,6 @@ protected:
   static BOOL CALLBACK screen_cb(HMONITOR mon, HDC, LPRECT r, LPARAM);
   BOOL screen_cb(HMONITOR mon, HDC, LPRECT r);
   int get_mouse_unscaled(int &mx, int &my);
-  void init_screen_scale_factors();
 
 public:
   Fl_WinAPI_Screen_Driver() : Fl_Screen_Driver() {
@@ -92,7 +91,7 @@ public:
   virtual void scale(int n, float f) {
     scale_of_screen[n] = f;
   }
-  virtual float desktop_scale_factor();
+  virtual void desktop_scale_factor();
 };
 
 

src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx

@@ -158,11 +158,6 @@ void Fl_WinAPI_Screen_Driver::init()
 }
 
 
-float Fl_WinAPI_Screen_Driver::desktop_scale_factor() {
-  return 0; //indicates each screen has already been assigned its scale factor value
-}
-
-
 void Fl_WinAPI_Screen_Driver::screen_work_area(int &X, int &Y, int &W, int &H, int n)
 {
   if (num_screens < 0) init();

src/drivers/X11/Fl_X11_Screen_Driver.H

@@ -55,7 +55,7 @@ public:
   virtual APP_SCALING_CAPABILITY rescalable() { return PER_SCREEN_APP_SCALING; }
   virtual float scale(int n) {return screens[n].scale;}
   virtual void scale(int n, float f) { screens[n].scale = f;}
-  virtual float desktop_scale_factor();
+  virtual void desktop_scale_factor();
   int screen_num_unscaled(int x, int y);
 #endif
   

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -1435,23 +1435,33 @@ static bool gnome_scale_factor(float& factor) {
 #endif // HAVE_DLSYM && HAVE_DLFCN_H
 
 
-// return the desktop's default scaling value
-float Fl_X11_Screen_Driver::desktop_scale_factor()
+// set the desktop's default scaling value
+void Fl_X11_Screen_Driver::desktop_scale_factor()
 {
+  float factor = 1;
+  bool doit = false;
   // First, try getting the Xft.dpi resource value
   char *s = XGetDefault(fl_display, "Xft", "dpi");
   if (s) {
     int dpi = 96;
     sscanf(s, "%d", &dpi);
-    return dpi / 96.;
-  }
-  float factor = 1;
-  if (!usemonitors_xml(factor, screens[0].width, screens[0].height)) {
+    factor = dpi / 96.;
+    doit = true;
+  } else {
+    screen_count(); // keep here
+    doit = usemonitors_xml(factor, screens[0].width, screens[0].height);
 #if HAVE_DLSYM && HAVE_DLFCN_H
-    gnome_scale_factor(factor);
+    if (!doit) {
+      doit = gnome_scale_factor(factor);
+    }
 #endif
   }
-  return factor;
+  if (doit) {
+    // checks to prevent potential crash (factor <= 0) or very large factors
+    if (factor < 0.25) factor = 0.25;
+    else if (factor > 10.0) factor = 10.0;
+    for (int i = 0; i < screen_count(); i++)  scale(i, factor);
+  }
 }
 
 #endif // USE_XFT