Use last display setup instead of recomputing for window_range

Fixes #4964

Author: mawww

src/main.cc

@@ -385,7 +385,7 @@ static const EnvVarDesc builtin_env_vars[] = { {
         "window_range", false,
         [](StringView name, const Context& context) -> Vector<String>
         {
-            auto setup = context.window().compute_display_setup(context);
+            const auto& setup = context.window().last_display_setup();
             return {format("{} {} {} {}", setup.first_line, setup.first_column,
                                           setup.line_count, 0)};
         }

src/window.cc

@@ -155,6 +155,7 @@ const DisplayBuffer& Window::update_display_buffer(const Context& context)
 
     set_position({setup.first_line, setup.first_column});
     m_last_setup = build_setup(context);
+    m_last_display_setup = setup;
 
     return m_display_buffer;
 }

src/window.hh

@@ -49,10 +49,12 @@ public:
     void clear_display_buffer();
     void run_resize_hook_ifn();
 
-    DisplaySetup compute_display_setup(const Context& context) const;
+    const DisplaySetup& last_display_setup() const { return m_last_display_setup; }
+
 private:
     Window(const Window&) = delete;
 
+    DisplaySetup compute_display_setup(const Context& context) const;
     void on_option_changed(const Option& option) override;
 
     friend class ClientManager;
@@ -68,6 +70,7 @@ private:
 
     Highlighters m_builtin_highlighters;
     bool m_resize_hook_pending = false;
+    DisplaySetup m_last_display_setup;
 
     struct Setup
     {