Fix messed up display when empty prompt.

spec/expression_editor_spec.cr

@@ -428,6 +428,39 @@ module Reply
                            "****>5"
     end
 
+    it "Don't mess up the terminal when the prompt is empty" do
+      editor = ExpressionEditor.new { "" }
+      editor.output = IO::Memory.new
+      editor.color = false
+      editor.height = 5
+      editor.width = 15
+
+      editor.update { editor << "Hello,\nWorld" }
+      editor.verify_output "\e[1G\e[J" \
+                           "Hello,\n" \
+                           "World"
+
+      editor.output = IO::Memory.new
+      editor.update { editor << '\n' }
+      editor.verify_output "\e[1A\e[1G\e[J" \
+                           "Hello,\n" \
+                           "World\n"
+
+      editor.output = IO::Memory.new
+      editor.update { editor << "1+1" }
+      editor.verify_output "\e[2A\e[1G\e[J" \
+                           "Hello,\n" \
+                           "World\n" \
+                           "1+1"
+
+      editor.output = IO::Memory.new
+      editor.update { editor << '\n' }
+      editor.verify_output "\e[2A\e[1G\e[J" \
+                           "Hello,\n" \
+                           "World\n" \
+                           "1+1\n"
+    end
+
     # TODO:
     # header
   end

src/expression_editor.cr

@@ -95,6 +95,7 @@ module Reply
     # Creates a new `ExpressionEditor` with the given *prompt*.
     def initialize(&@prompt : Int32, Bool -> String)
       @prompt_size = @prompt.call(0, false).size # uncolorized size
+      @prompt_size = 1 if @prompt_size == 0
     end
 
     # Sets a `Proc` allowing to display a header above the prompt. (used by auto-completion)
@@ -753,6 +754,8 @@ module Reply
 
     private def print_prompt(io, line_index)
       line_prompt_size = @prompt.call(line_index, false).size # uncolorized size
+      line_prompt_size = 1 if line_prompt_size == 0
+
       @prompt_size = {line_prompt_size, @prompt_size}.max
       io.print @prompt.call(line_index, color?)