Always use alternate sceen on alt-d (#988)

Author: tompng

lib/irb/easter-egg.rb

@@ -125,7 +125,6 @@ def render_frame(i)
             canvas = Canvas.new(Reline.get_screen_size)
           end
           ruby_model = RubyModel.new
-          print "\e[?1049h"
           0.step do |i| # TODO (0..).each needs Ruby 2.6 or later
             buff = canvas.draw do
               ruby_model.render_frame(i) do |p1, p2|

lib/irb/input-method.rb

@@ -348,9 +348,15 @@ def show_doc_dialog_proc
           if show_easter_egg
             IRB.__send__(:easter_egg)
           else
+            # RDoc::RI::Driver#display_names uses pager command internally.
+            # Some pager command like `more` doesn't use alternate screen
+            # so we need to turn on and off alternate screen manually.
             begin
+              print "\e[?1049h"
               driver.display_names([name])
             rescue RDoc::RI::Driver::NotFoundError
+            ensure
+              print "\e[?1049l"
             end
           end
         end