add：resizable

main.cpp

@@ -1,5 +1,6 @@
 #include <FL/Fl.H>
 #include <FL/Fl_Box.H>
+#include <FL/Fl_Button.H>
 #include <FL/Fl_Pack.H>
 #include <FL/Fl_Window.H>
 
@@ -40,7 +41,26 @@ int demo1(int argc, char** argv)
     return Fl::run();
 }
 
+int demo2()
+{
+    auto* dialog = new Fl_Window(300, 100);
+    auto* icon = new Fl_Box(0, 0, 50, 50, "!");
+    auto* text = new Fl_Box(50, 0, 250, 40, "Out of Memory Error");
+    auto* btns = new Fl_Group(50, 50, 250, 50);   // parent group
+    auto* darn = new Fl_Button(200, 50, 100, 50, "Darn!");
+    auto* R = new Fl_Box(50, 50, 150, 50);   // "invisible" box "R"
+    R->hide();                               // make sure it's invisible
+    btns->resizable(R);                      // make "R" parent group resizable
+    btns->end();
+    dialog->resizable(darn);
+    dialog->end();
+    dialog->show();
+    // 运行事件循环
+    return Fl::run();
+}
+
 int main(int argc, char** argv)
 {
-    demo1(argc, argv);
+    // demo1(argc, argv);
+    demo2();
 }