Dialog rendering should Scroll down only when needed

Author: tompng

lib/reline/line_editor.rb

@@ -719,8 +719,10 @@ def add_dialog_proc(name, p, context = nil)
     ymax = ymax.clamp(screen_y_range.begin, screen_y_range.end)
     dialog_y = @first_line_started_from + @started_from
     cursor_y = dialog_y
-    scroll_down(ymax - cursor_y)
-    move_cursor_up(ymax - cursor_y)
+    if @highest_in_all < ymax
+      scroll_down(ymax - cursor_y)
+      move_cursor_up(ymax - cursor_y)
+    end
     (ymin..ymax).each do |y|
       move_cursor_down(y - cursor_y)
       cursor_y = y

test/reline/yamatanooroti/test_rendering.rb

@@ -916,6 +916,20 @@ def test_simple_dialog_at_right_edge
       EOC
     end
 
+    def test_simple_dialog_with_scroll_screen
+      start_terminal(5, 50, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --dialog simple}, startup_message: 'Multiline REPL.')
+      write("if 1\n  2\n  3\n  4\n  5\n  6")
+      write("\C-p\C-n\C-p\C-p\C-p#")
+      close
+      assert_screen(<<~'EOC')
+        prompt>   2
+        prompt>   3#
+        prompt>   4
+        prompt>   5
+        prompt>   6 Ruby is...
+      EOC
+    end
+
     def test_autocomplete_at_bottom
       start_terminal(15, 50, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --autocomplete}, startup_message: 'Multiline REPL.')
       write('def hoge' + "\C-m" * 10 + "end\C-p  ")