Fix "Reset Scale" notification when there is enough workarea available

Instead of using the window size, we can use the available workarea to
know that the user can resize the main window to its full size and
there will be enough room to display some common dialogs like the
Preferences one.

src/app/ui/main_window.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2021  Igara Studio S.A.
+// Copyright (C) 2018-2022  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -377,12 +377,21 @@ void MainWindow::onResize(ui::ResizeEvent& ev)
   app::gen::MainWindow::onResize(ev);
 
   os::Window* display = manager()->display();
-  if ((display) &&
-      (display->scale()*ui::guiscale() > 2) &&
-      (!m_scalePanic) &&
-      (ui::display_w()/ui::guiscale() < 320 ||
-       ui::display_h()/ui::guiscale() < 260)) {
-    showNotification(m_scalePanic = new ScreenScalePanic);
+  if (display && display->screen()) {
+    const int scale = display->scale()*ui::guiscale();
+
+    // We can check for the available workarea to know that the user
+    // can resize the window to its full size and there will be enough
+    // room to display some common dialogs like (for example) the
+    // Preferences dialog.
+    if ((scale > 2) &&
+        (!m_scalePanic)) {
+      const gfx::Size wa = display->screen()->workarea().size();
+      if ((wa.w / scale < 256 ||
+           wa.h / scale < 256)) {
+        showNotification(m_scalePanic = new ScreenScalePanic);
+      }
+    }
   }
 }