LibGUI: Defer line reflow during text insertion

Add a deferral counter and defer reflowing the visual lines until the
counter is at zero. Use this to defer reflow when inserting text.

This fixes glacial slowdown while paste large amounts of text.

Libraries/LibGUI/TextEditor.cpp

@@ -1197,6 +1197,7 @@ void TextEditor::delete_selection()
 
 void TextEditor::insert_at_cursor_or_replace_selection(const StringView& text)
 {
+    ReflowDeferrer defer(*this);
     ASSERT(!is_readonly());
     if (has_selection())
         delete_selection();
@@ -1224,6 +1225,7 @@ void TextEditor::paste()
 {
     if (is_readonly())
         return;
+
     auto paste_text = Clipboard::the().data();
     printf("Paste: \"%s\"\n", paste_text.characters());
 
@@ -1231,6 +1233,20 @@ void TextEditor::paste()
     insert_at_cursor_or_replace_selection(paste_text);
 }
 
+void TextEditor::defer_reflow()
+{
+    ++m_reflow_deferred;
+}
+
+void TextEditor::undefer_reflow()
+{
+    ASSERT(m_reflow_deferred);
+    if (!--m_reflow_deferred) {
+        if (m_reflow_requested)
+            recompute_all_visual_lines();
+    }
+}
+
 void TextEditor::enter_event(Core::Event&)
 {
     ASSERT(window());
@@ -1359,6 +1375,11 @@ void TextEditor::clear_selection()
 
 void TextEditor::recompute_all_visual_lines()
 {
+    if (m_reflow_deferred) {
+        m_reflow_requested = true;
+        return;
+    }
+
     int y_offset = 0;
     for (size_t line_index = 0; line_index < line_count(); ++line_index) {
         recompute_visual_lines(line_index);