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);

Libraries/LibGUI/TextEditor.h

@@ -172,6 +172,24 @@ private:
     void did_change();
     int fixed_glyph_width() const;
 
+    void defer_reflow();
+    void undefer_reflow();
+
+    class ReflowDeferrer {
+    public:
+        ReflowDeferrer(TextEditor& editor)
+            : m_editor(editor)
+        {
+            m_editor.defer_reflow();
+        }
+        ~ReflowDeferrer()
+        {
+            m_editor.undefer_reflow();
+        }
+    private:
+        TextEditor& m_editor;
+    };
+
     Gfx::Rect line_content_rect(size_t item_index) const;
     Gfx::Rect line_widget_rect(size_t line_index) const;
     Gfx::Rect cursor_content_rect() const;
@@ -238,6 +256,9 @@ private:
     Core::ElapsedTimer m_triple_click_timer;
     NonnullRefPtrVector<Action> m_custom_context_menu_actions;
 
+    size_t m_reflow_deferred { 0 };
+    size_t m_reflow_requested { 0 };
+
     RefPtr<TextDocument> m_document;
 
     template<typename Callback>