bugfix in previous commit

I almost pushed this to production. That would have been catastrophic;
the very first keystroke anyone typed into the editor would have failed.

And in the process, this fixes the next bug on my TODO list! Paste on
first line wasn't working. Now it is.

text.lua

@@ -152,6 +152,7 @@ function Text.insert_at_cursor(t)
   else
     byte_offset = 1
   end
+--?   print(Screen_top1.line, Screen_top1.pos, Cursor1.line, Cursor1.pos, Screen_bottom1.line, Screen_bottom1.pos)
   Lines[Cursor1.line].data = string.sub(Lines[Cursor1.line].data, 1, byte_offset-1)..t..string.sub(Lines[Cursor1.line].data, byte_offset)
   Lines[Cursor1.line].fragments = nil
   Lines[Cursor1.line].screen_line_starting_pos = nil
@@ -160,6 +161,7 @@ function Text.insert_at_cursor(t)
   if scroll_down then
     Text.populate_screen_line_starting_pos(Cursor1.line)
     Text.snap_cursor_to_bottom_of_screen()
+--?     print('=>', Screen_top1.line, Screen_top1.pos, Cursor1.line, Cursor1.pos, Screen_bottom1.line, Screen_bottom1.pos)
   end
 end
 
@@ -624,6 +626,7 @@ end
 
 function Text.snap_cursor_to_bottom_of_screen()
   local top2 = Text.to2(Cursor1)
+  top2.screen_pos = 1  -- start of screen line
 --?   print('cursor pos '..tostring(Cursor1.pos)..' is on the #'..tostring(top2.screen_line)..' screen line down')
   local y = App.screen.height - Line_height
   -- duplicate some logic from love.draw

text_tests.lua

@@ -1,6 +1,16 @@
 -- major tests for text editing flows
 -- This still isn't quite as thorough as I'd like.
 
+function test_insert_first_character()
+  io.write('\ntest_insert_first_character')
+  App.screen.init{width=120, height=60}
+  Lines = load_array{}
+  App.draw()
+  App.run_after_textinput('a')
+  local y = Margin_top
+  App.screen.check(y, 'a', 'F - test_insert_first_character/screen:1')
+end
+
 function test_draw_text()
   io.write('\ntest_draw_text')
   App.screen.init{width=120, height=60}