keep all text cache writes inside text.lua

edit.lua

@@ -130,10 +130,9 @@ function edit.draw(State)
     State.screen_bottom1.line = line_index
     if line.mode == 'text' then
 --?       print('text.draw', y, line_index)
-      line.starty = y
-      line.startpos = 1
+      local startpos = 1
       if line_index == State.screen_top1.line then
-        line.startpos = State.screen_top1.pos
+        startpos = State.screen_top1.pos
       end
       if line.data == '' then
         -- button to insert new drawing
@@ -150,7 +149,7 @@ function edit.draw(State)
                      end,
         })
       end
-      y, State.screen_bottom1.pos = Text.draw(State, line_index)
+      y, State.screen_bottom1.pos = Text.draw(State, line_index, y, startpos)
       y = y + State.line_height
 --?       print('=> y', y)
     elseif line.mode == 'drawing' then

text.lua

@@ -6,16 +6,16 @@ require 'select'
 require 'undo'
 require 'text_tests'
 
--- return values:
---  y coordinate drawn until in px
---  position of start of final screen line drawn
-function Text.draw(State, line_index)
+-- draw a line starting from startpos to screen at y between State.left and State.right
+-- return the final y, and position of start of final screen line drawn
+function Text.draw(State, line_index, y, startpos)
 --?   print('text.draw', line_index)
   App.color(Text_color)
   local line = State.lines[line_index]
+  line.starty = y
+  line.startpos = startpos
   -- wrap long lines
   local x = State.left
-  local y = line.starty
   local pos = 1
   local screen_line_starting_pos = 1
   if line.fragments == nil then