LibGUI: Prevent CenterWithinParent Dialogs from appearing offscreen

When a dialog is created the position is checked against the Desktop's
rect and repositioned to be entirely visible. If the dialog is larger
than the desktop's rect it is just centered.

Userland/Libraries/LibGUI/Dialog.cpp

@@ -9,6 +9,7 @@
 #include <LibGUI/Desktop.h>
 #include <LibGUI/Dialog.h>
 #include <LibGUI/Event.h>
+#include <LibGfx/Palette.h>
 
 namespace GUI {
 
@@ -39,7 +40,27 @@ Dialog::ExecResult Dialog::exec()
         if (parent() && is<Window>(parent())) {
             auto& parent_window = *static_cast<Window*>(parent());
             if (parent_window.is_visible()) {
+                // Check the dialog's positiom against the Desktop's rect and reposition it to be entirely visible.
+                // If the dialog is larger than the desktop's rect just center it.
                 window_rect.center_within(parent_window.rect());
+                if (window_rect.size().width() < desktop_rect.size().width() && window_rect.size().height() < desktop_rect.size().height()) {
+                    auto taskbar_top_y = desktop_rect.bottom() - Desktop::the().taskbar_height();
+                    auto palette = GUI::Application::the()->palette();
+                    auto border_thickness = palette.window_border_thickness();
+                    auto top_border_title_thickness = border_thickness + palette.window_title_height();
+                    if (window_rect.top() < top_border_title_thickness) {
+                        window_rect.set_y(top_border_title_thickness);
+                    }
+                    if (window_rect.right() + border_thickness > desktop_rect.right()) {
+                        window_rect.translate_by((window_rect.right() + border_thickness - desktop_rect.right()) * -1, 0);
+                    }
+                    if (window_rect.bottom() + border_thickness > taskbar_top_y) {
+                        window_rect.translate_by(0, (window_rect.bottom() + border_thickness - taskbar_top_y) * -1);
+                    }
+                    if (window_rect.left() - border_thickness < 0) {
+                        window_rect.set_x(0 + border_thickness);
+                    }
+                }
                 break;
             }
         }