Avoid repeated TTY readline output with TERM=dumb

lib/reline/io/dumb.rb

@@ -9,9 +9,11 @@ def initialize(encoding: nil)
     @input = STDIN
     @output = STDOUT
     @buf = []
+    @output_buffer = nil
     @pasting = false
     @encoding = encoding
     @screen_size = [24, 80]
+    @first_render = true
   end
 
   def dumb?
@@ -43,11 +45,29 @@ def with_raw_input
   end
 
   def write(string)
-    @output.write(string)
+    if @output_buffer
+      @output_buffer << string
+    else
+      @output.write(string)
+    end
   end
 
   def buffered_output
+    return yield unless tty?
+
+    @output_buffer = +''
     yield
+
+    if @first_render || @output_buffer.end_with?("\n")
+      @output.write(@output_buffer)
+      @first_render = false
+    end
+  ensure
+    @output_buffer = nil
+  end
+
+  private def tty?
+    @input.respond_to?(:tty?) && @input.tty? && @output.respond_to?(:tty?) && @output.tty?
   end
 
   def getc(_timeout_second)
@@ -113,6 +133,7 @@ def in_pasting?
   end
 
   def prep
+    @first_render = true
   end
 
   def deprep(otio)

test/reline/test_reline.rb

@@ -428,6 +428,26 @@ def test_readline_returns_nil_on_piped_stdin_eof
     assert_include(out, { result: nil }.inspect)
   end
 
+  def test_dumb_tty_buffered_output_keeps_first_and_finished_renders
+    input = StringIO.new
+    output = StringIO.new
+
+    [input, output].each do |io|
+      def io.tty?
+        true
+      end
+    end
+    io_gate = Reline::Dumb.new
+    io_gate.input = input
+    io_gate.output = output
+
+    io_gate.buffered_output { io_gate.write("first render") }
+    io_gate.buffered_output { io_gate.write("rerender") }
+    io_gate.buffered_output { io_gate.write("finished render\n") }
+
+    assert_equal("first renderfinished render\n", output.string)
+  end
+
   def test_read_eof_returns_input
     pend if win?
     lib = File.expand_path("../../lib", __dir__)