Fix "Windows rapidly changing scales near edge between screens" - cont'd (#1439)

src/Fl_win32.cxx

@@ -1238,6 +1238,16 @@ static BOOL CALLBACK child_window_cb(HWND child_xid, LPARAM data) {
 }
 
 
+static struct win_w_h {
+  Fl_Window *win;
+  int W, H;
+} moved_win_data; // FLTK size of window at start of the window move operation
+
+static void delayed_size(void *) { // performed after end of win move operation or change of screen
+  moved_win_data.win->size(moved_win_data.W, moved_win_data.H);
+}
+
+
 static LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
 
   // Copy the message to fl_msg so add_handler code can see it.
@@ -1276,7 +1286,11 @@ static LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lPar
             sd->update_scaling_capability();
           } else if (ns != old_ns) {
             // jump window with Windows+Shift+L|R-arrow to other screen with other DPI
-            if (ns >= 0) Fl_Window_Driver::driver(window)->screen_num(ns);
+            if (ns >= 0) {
+              //fprintf(stderr,"WM_DPICHANGED ns %d --> %d moving_window=%d\n",old_ns,ns,moving_window);
+              Fl_Window_Driver::driver(window)->screen_num(ns);
+              if (!moving_window) moved_win_data = {window, window->w(), window->h()};
+            }
             UINT flags = SWP_NOSENDCHANGING | SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOOWNERZORDER | SWP_NOCOPYBITS;
             SetWindowPos(hWnd, NULL, lParam_rect->left, lParam_rect->top,
                          lParam_rect->right - lParam_rect->left,
@@ -1288,6 +1302,7 @@ static LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lPar
               }
               scale = Fl::screen_driver()->scale(ns);
               EnumChildWindows(hWnd, child_window_cb, (LPARAM)&scale);
+              if (!moving_window) Fl::add_timeout(0, (Fl_Timeout_Handler)delayed_size, NULL);
             }
           }
         }
@@ -1764,11 +1779,20 @@ content  key    keyboard layout
         return 0;
 
       case WM_MOVING:
+        //fprintf(stderr,"WM_MOVING\n");
         moving_window = true;
         return 1;
 
       case WM_CAPTURECHANGED:
-        moving_window = false;
+        //fprintf(stderr,"WM_CAPTURECHANGED moving_window=%d\n",moving_window);
+        if (!moving_window) {
+          moved_win_data.win = window;
+          moved_win_data.W = window->w();
+          moved_win_data.H = window->h();
+        } else {
+          moving_window = false;
+          Fl::add_timeout(0, (Fl_Timeout_Handler)delayed_size, NULL);
+        }
         resize_bug_fix = 0;
         return 0;
 
@@ -1779,6 +1803,27 @@ content  key    keyboard layout
         if (moving_window) resize_bug_fix = window;
         POINTS pts = MAKEPOINTS(lParam);
         Fl_Window_Driver *wd = Fl_Window_Driver::driver(window);
+        Fl_WinAPI_Screen_Driver *sd = (Fl_WinAPI_Screen_Driver *)Fl::screen_driver();
+        // detect when window centre changes screen
+        int olds = wd->screen_num();
+        // Issue #1097: when a fullscreen window is restored to its size, it receives first a WM_MOVE
+        // and then a WM_SIZE, so it still has its fullscreen size at the WM_MOVE event, which defeats
+        // using window->w()|h() to compute the center of the (small) window. We detect this situation
+        // with condition: !window->fullscreen_active() && *wd->no_fullscreen_w()
+        // and use *wd->no_fullscreen_w()|h() instead of window->w()|h().
+        int trueW = window->w(), trueH = window->h();
+        if (!window->fullscreen_active() && *wd->no_fullscreen_w()) {
+          trueW = *wd->no_fullscreen_w(); trueH = *wd->no_fullscreen_h();
+        }
+        int news = sd->screen_num_unscaled(pts.x + int(trueW * scale / 2), pts.y + int(trueH * scale / 2));
+        //fprintf(stderr,"WM_MOVE to %dx%d ns %d--> %d scale=%g\n",pts.x,pts.y,olds,news,sd->scale(news>=0?news:olds));
+        if (news == -1)
+          news = olds;
+        else if (news != olds) {
+          wd->screen_num(news);
+          scale = sd->scale(news);
+          window->redraw();
+        }
         wd->x(int(round(pts.x / scale)));
         wd->y(int(round(pts.y / scale)));
         }