From a8f0e4d56e70013803c09943ffb33e4d86c768fa Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Wed, 27 May 2020 19:18:40 +0200
Subject: [PATCH] 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 | 21 +++++++++++++++++++++
 Libraries/LibGUI/TextEditor.h   | 21 +++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/Libraries/LibGUI/TextEditor.cpp b/Libraries/LibGUI/TextEditor.cpp
index 85f166d35b..5de7dab05a 100644
--- a/Libraries/LibGUI/TextEditor.cpp
+++ b/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);
diff --git a/Libraries/LibGUI/TextEditor.h b/Libraries/LibGUI/TextEditor.h
index f5fa134b4c..a6138ecde9 100644
--- a/Libraries/LibGUI/TextEditor.h
+++ b/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>