LibWeb: Refuse to recursively execute .execCommand()

Spec issue:

  https://github.com/w3c/editing/issues/477

Libraries/LibWeb/DOM/Document.h

@@ -1101,6 +1101,8 @@ private:
 
     GC::Ref<EditingHostManager> m_editing_host_manager;
 
+    bool m_inside_exec_command { false };
+
     // https://w3c.github.io/editing/docs/execCommand/#default-single-line-container-name
     FlyString m_default_single_line_container_name { HTML::TagNames::div };
 

Libraries/LibWeb/Editing/ExecCommand.cpp

@@ -20,6 +20,12 @@ WebIDL::ExceptionOr<bool> Document::exec_command(FlyString const& command, [[may
     if (!is_html_document())
         return WebIDL::InvalidStateError::create(realm(), "execCommand is only supported on HTML documents"_string);
 
+    // AD-HOC: All major browsers refuse to recursively execute execCommand() (e.g. inside input event handlers).
+    if (m_inside_exec_command)
+        return false;
+    ScopeGuard guard_recursion = [&] { m_inside_exec_command = false; };
+    m_inside_exec_command = true;
+
     // 1. If only one argument was provided, let show UI be false.
     // 2. If only one or two arguments were provided, let value be the empty string.
     // NOTE: these steps are dealt by the default values for both show_ui and value

Tests/LibWeb/Text/expected/Editing/execCommand-no-recursion.txt

@@ -0,0 +1,4 @@
+input event
+inner result: false
+outer result: true
+foo

Tests/LibWeb/Text/input/Editing/execCommand-no-recursion.html

@@ -0,0 +1,20 @@
+<script src="../include.js"></script>
+<div contenteditable="true"></div>
+<script>
+    test(() => {
+        const range = document.createRange();
+        getSelection().addRange(range);
+
+        const divElm = document.querySelector('div');
+        range.setStart(divElm, 0);
+        range.setEnd(divElm, 0);
+
+        divElm.addEventListener('input', () => {
+            println('input event');
+            println(`inner result: ${document.execCommand('insertText', false, 'bar')}`);
+        });
+
+        println(`outer result: ${document.execCommand('insertText', false, 'foo')}`);
+        println(divElm.innerHTML);
+    });
+</script>