snapshot - no, that's all wrong

I've been only thinking about up arrow when cursor is at top of screen.
Hopefully this is better.

main.lua

@@ -38,7 +38,7 @@ require 'icons'
 -- once, and read them passively thereafter.
 Lines = {{mode='text', data=''}}
 Cursor_line = 1
-Cursor_pos = 1  -- in Unicode codepoints, from 1 to utf8.len(line) + 1
+Cursor_pos = 15  -- in Unicode codepoints, from 1 to utf8.len(line) + 1
 
 Screen_width, Screen_height, Screen_flags = 0, 0, nil
 
@@ -47,7 +47,7 @@ Cursor_x, Cursor_y = 0, 0  -- in pixels
 -- scrolling support
 Screen_top_line = 1
 Screen_bottom_line = 1
-Top_screen_line_starting_pos = 1  -- when top of screen starts in between a wrapped line
+Top_screen_line_starting_pos = 6  -- when top of screen starts in between a wrapped line
 Bottom_screen_line_starting_pos = 1  -- when bottom of screen starts in between a wrapped line
 
 Current_drawing_mode = 'line'
@@ -61,15 +61,17 @@ Filename = 'lines.txt'
 
 function love.load(arg)
   -- maximize window
-  love.window.setMode(0, 0)  -- maximize
-  Screen_width, Screen_height, Screen_flags = love.window.getMode()
-  -- shrink slightly to account for window decoration
-  Screen_width = Screen_width-100
-  Screen_height = Screen_height-100
+--?   love.window.setMode(0, 0)  -- maximize
+--?   Screen_width, Screen_height, Screen_flags = love.window.getMode()
+--?   -- shrink slightly to account for window decoration
+--?   Screen_width = Screen_width-100
+--?   Screen_height = Screen_height-100
+  Screen_width = 120
+  Screen_height = 200
   love.window.setMode(Screen_width, Screen_height)
   love.window.setTitle('Text with Lines')
---?   Line_width = 100
-  Line_width = math.floor(Screen_width/2/40)*40
+  Line_width = 100
+--?   Line_width = math.floor(Screen_width/2/40)*40
   love.keyboard.setTextInput(true)  -- bring up keyboard on touch screen
   love.keyboard.setKeyRepeat(true)
   if #arg > 0 then

manual_tests

@@ -22,7 +22,7 @@ scrolling:
   cursor remains on screen
   cursor remains on text line
   'up' arrow with cursor at top of screen scrolls up one line (drawings still fully in or out)
-    if top line wrapped before, it scrolls up by only one screen line
+    if cursor line wrapped before, it scrolls up by only one screen line
     if previous line (above top of screen) wrapped, it scrolls up by only one screen line
   'down' arrow with cursor at bottom of screen scrolls down one line (drawings still fully in or out)
-    if top line wrapped before, it scrolls down by only one screen line
+    if cursor line wrapped before, it scrolls down by only one screen line

text.lua

@@ -219,7 +219,9 @@ function Text.keychord_pressed(chord)
     save_to_disk(Lines, Filename)
   elseif chord == 'up' then
     assert(Lines[Cursor_line].mode == 'text')
-    if Top_screen_line_starting_pos == 1 then
+    local screen_line_index,screen_line_starting_pos = Text.pos_at_start_of_cursor_screen_line()
+    if screen_line_starting_pos == 1 then
+      print('cursor is at first screen line of its line')
       -- top line is done; skip to previous text line
       local new_cursor_line = Cursor_line
       while new_cursor_line > 1 do
@@ -233,7 +235,10 @@ function Text.keychord_pressed(chord)
           end
           local screen_line_starting_pos = Lines[Cursor_line].screen_line_starting_pos
           screen_line_starting_pos = screen_line_starting_pos[#screen_line_starting_pos]
-          Top_screen_line_starting_pos = screen_line_starting_pos
+          print('previous screen line starts at '..tostring(screen_line_starting_pos)..' of its line')
+          if Screen_top_line == Cursor_line and Top_screen_line_starting_pos == screen_line_starting_pos then
+            Top_screen_line_starting_pos = screen_line_starting_pos
+          end
           local s = string.sub(Lines[Cursor_line].data, screen_line_starting_pos)
           Cursor_pos = screen_line_starting_pos + Text.nearest_cursor_pos(s, old_x) - 1
           break
@@ -244,11 +249,17 @@ function Text.keychord_pressed(chord)
       end
     else
       -- scroll up just one screen line in current line
-      local screen_line_index = table.find(Lines[Cursor_line].screen_line_starting_pos, Top_screen_line_starting_pos)
+      print('cursor is NOT at first screen line of its line')
       assert(screen_line_index > 1)
-      Top_screen_line_starting_pos = Lines[Cursor_line].screen_line_starting_pos[screen_line_index-1]
-      local s = string.sub(Lines[Cursor_line].data, Top_screen_line_starting_pos)
-      Cursor_pos = Text.nearest_cursor_pos(s, Cursor_x)
+      new_screen_line_starting_pos = Lines[Cursor_line].screen_line_starting_pos[screen_line_index-1]
+      print('switching pos of screen line at cursor from '..tostring(screen_line_starting_pos)..' to '..tostring(new_screen_line_starting_pos))
+      if Screen_top_line == Cursor_line and Top_screen_line_starting_pos == screen_line_starting_pos then
+        Top_screen_line_starting_pos = new_screen_line_starting_pos
+        print('also setting pos of top of screen to '..tostring(Top_screen_line_starting_pos))
+      end
+      local s = string.sub(Lines[Cursor_line].data, new_screen_line_starting_pos)
+      Cursor_pos = new_screen_line_starting_pos + Text.nearest_cursor_pos(s, Cursor_x) - 1
+      print('cursor pos is now '..tostring(Cursor_pos))
     end
   elseif chord == 'down' then
     assert(Lines[Cursor_line].mode == 'text')
@@ -269,6 +280,19 @@ function Text.keychord_pressed(chord)
   end
 end
 
+function Text.pos_at_start_of_cursor_screen_line()
+  if Lines[Cursor_line].screen_line_starting_pos == nil then
+    return 1,1
+  end
+  for i=#Lines[Cursor_line].screen_line_starting_pos,1,-1 do
+    local spos = Lines[Cursor_line].screen_line_starting_pos[i]
+    if spos <= Cursor_pos then
+      return i,spos
+    end
+  end
+  assert(false)
+end
+
 function Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()
   while Cursor_line <= #Lines do
     if Lines[Cursor_line].mode == 'text' then