LibWeb: Add helpers to convert a MouseButton to/from its DOM event code

Move the helper to convert MouseButton to its DOM event code to a public
header. And add a helper to convert in the opposite direction.

Userland/Libraries/LibWeb/UIEvents/MouseButton.h

@@ -6,7 +6,9 @@
 
 #pragma once
 
+#include <AK/Assertions.h>
 #include <AK/EnumBits.h>
+#include <AK/Types.h>
 
 namespace Web::UIEvents {
 
@@ -21,4 +23,39 @@ enum MouseButton : u8 {
 
 AK_ENUM_BITWISE_OPERATORS(MouseButton);
 
+// https://www.w3.org/TR/uievents/#dom-mouseevent-button
+constexpr i16 mouse_button_to_button_code(MouseButton button)
+{
+    switch (button) {
+    case MouseButton::Primary:
+        return 0;
+    case MouseButton::Middle:
+        return 1;
+    case MouseButton::Secondary:
+        return 2;
+    case MouseButton::Backward:
+        return 3;
+    case MouseButton::Forward:
+        return 4;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+// https://www.w3.org/TR/uievents/#dom-mouseevent-button
+constexpr MouseButton button_code_to_mouse_button(i16 button)
+{
+    if (button == 0)
+        return MouseButton::Primary;
+    if (button == 1)
+        return MouseButton::Middle;
+    if (button == 2)
+        return MouseButton::Secondary;
+    if (button == 3)
+        return MouseButton::Backward;
+    if (button == 4)
+        return MouseButton::Forward;
+    return MouseButton::None;
+}
+
 }

Userland/Libraries/LibWeb/UIEvents/MouseEvent.cpp

@@ -123,25 +123,6 @@ void MouseEvent::init_mouse_event(String const& type, bool bubbles, bool cancela
     m_related_target = related_target;
 }
 
-// https://www.w3.org/TR/uievents/#dom-mouseevent-button
-static i16 determine_button(unsigned mouse_button)
-{
-    switch (mouse_button) {
-    case MouseButton::Primary:
-        return 0;
-    case MouseButton::Middle:
-        return 1;
-    case MouseButton::Secondary:
-        return 2;
-    case MouseButton::Backward:
-        return 3;
-    case MouseButton::Forward:
-        return 4;
-    default:
-        VERIFY_NOT_REACHED();
-    }
-}
-
 JS::NonnullGCPtr<MouseEvent> MouseEvent::create(JS::Realm& realm, FlyString const& event_name, MouseEventInit const& event_init, double page_x, double page_y, double offset_x, double offset_y)
 {
     return realm.heap().allocate<MouseEvent>(realm, realm, event_name, event_init, page_x, page_y, offset_x, offset_y);
@@ -167,7 +148,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<MouseEvent>> MouseEvent::create_from_platfo
         event_init.movement_x = movement.value().x().to_double();
         event_init.movement_y = movement.value().y().to_double();
     }
-    event_init.button = determine_button(button);
+    event_init.button = mouse_button_to_button_code(static_cast<MouseButton>(button));
     event_init.buttons = buttons;
     auto event = MouseEvent::create(realm, event_name, event_init, page.x().to_double(), page.y().to_double(), offset.x().to_double(), offset.y().to_double());
     event->set_is_trusted(true);