bugfix: lines that aren't drawn from the start

I can't believe I didn't catch this until now. All I had to do is open
MobyDick.markdown from https://www.hogbaysoftware.com/posts/moby-dick-workout,
press page-down and click on the top screen line (or any screen line
containing the same line as the top screen line). Easy to catch with any
file containing lots of overly long lines, as happens in particular at
the start of Moby Dick.

I _had_ seen this problem before, but it seemed to disappear after
unrelated changes, and I convinced myself I'd fixed it as a side-effect.
The bug just failed to manifest if the top line happened to start at the
top of the screen. Scroll down a few pages in Moby Dick and the dialogue
starts and line length drops precipitously.

main.lua

@@ -20,7 +20,8 @@ function App.initialize_globals()
 -- a text is a table with:
 --    mode = 'text',
 --    string data,
---    a (y) coord in pixels (updated while painting screen),
+--    startpos, the index of data the line starts rendering from (if currently on screen), can only be >1 for topmost line on screen
+--    starty, the y coord in pixels
 --    some cached data that's blown away and recomputed when data changes:
 --      fragments: snippets of rendered love.graphics.Text, guaranteed to not wrap
 --      screen_line_starting_pos: optional array of grapheme indices if it wraps over more than one screen line
@@ -275,7 +276,11 @@ function App.draw()
         y = y + Drawing.pixels(line.h) + Drawing_padding_bottom
       else
 --?         print('text')
-        line.y = y
+        line.starty = y
+        line.startpos = 1
+        if line_index == Screen_top1.line then
+          line.startpos = Screen_top1.pos
+        end
         y, Screen_bottom1.pos = Text.draw(line, Line_width, line_index)
         y = y + Line_height
 --?         print('=> y', y)

text.lua

@@ -17,7 +17,7 @@ function Text.draw(line, line_width, line_index)
 --?   love.graphics.line(Line_width,0, Line_width,App.screen.height)
   -- wrap long lines
   local x = Margin_left
-  local y = line.y
+  local y = line.starty
   local pos = 1
   local screen_line_starting_pos = 1
   if line.fragments == nil then
@@ -659,11 +659,11 @@ function Text.snap_cursor_to_bottom_of_screen()
 end
 
 function Text.in_line(line_index,line, x,y)
-  if line.y == nil then return false end  -- outside current page
+  if line.starty == nil then return false end  -- outside current page
   if x < Margin_left then return false end
-  if y < line.y then return false end
+  if y < line.starty then return false end
   Text.populate_screen_line_starting_pos(line_index)
-  return y < line.y + #line.screen_line_starting_pos * Line_height
+  return y < line.starty + Line_height*(#line.screen_line_starting_pos - Text.screen_line_index(line, line.startpos) + 1)
 end
 
 -- convert mx,my in pixels to schema-1 coordinates
@@ -672,11 +672,13 @@ function Text.to_pos_on_line(line, mx, my)
   if line.fragments == nil then
     Text.compute_fragments(line, Line_width)
   end
-  assert(my >= line.y)
+  assert(my >= line.starty)
   -- duplicate some logic from Text.draw
-  local y = line.y
-  for screen_line_index,screen_line_starting_pos in ipairs(line.screen_line_starting_pos) do
-      local screen_line_starting_byte_offset = Text.offset(line.data, screen_line_starting_pos)
+  local y = line.starty
+  local start_screen_line_index = Text.screen_line_index(line, line.startpos)
+  for screen_line_index = start_screen_line_index,#line.screen_line_starting_pos do
+    local screen_line_starting_pos = line.screen_line_starting_pos[screen_line_index]
+    local screen_line_starting_byte_offset = Text.offset(line.data, screen_line_starting_pos)
 --?     print('iter', y, screen_line_index, screen_line_starting_pos, string.sub(line.data, screen_line_starting_byte_offset))
     local nexty = y + Line_height
     if my < nexty then
@@ -725,6 +727,14 @@ function Text.screen_line_width(line, i)
   return App.width(screen_line_text)
 end
 
+function Text.screen_line_index(line, pos)
+  for i = #line.screen_line_starting_pos,1,-1 do
+    if line.screen_line_starting_pos[i] <= pos then
+      return i
+    end
+  end
+end
+
 function Text.nearest_cursor_pos(line, x)  -- x includes left margin
   if x == 0 then
     return 1
@@ -909,7 +919,8 @@ end
 function Text.redraw_all()
 --?   print('clearing fragments')
   for _,line in ipairs(Lines) do
-    line.y = nil
+    line.starty = nil
+    line.startpos = nil
     Text.clear_cache(line)
   end
 end

text_tests.lua

@@ -184,6 +184,27 @@ function test_click_on_wrapping_line()
   check_eq(Cursor1.pos, 13, 'F - test_click_on_wrapping_line/cursor:pos')
 end
 
+function test_click_on_wrapping_line_rendered_from_partway_at_top_of_screen()
+  io.write('\ntest_click_on_wrapping_line_rendered_from_partway_at_top_of_screen')
+  -- display a wrapping line from its second screen line
+  App.screen.init{width=80, height=80}
+                  --  12345678901234
+  Lines = load_array{"madam I'm adam"}
+  Line_width = 75
+  Cursor1 = {line=1, pos=8}
+  Screen_top1 = {line=1, pos=7}
+  Screen_bottom1 = {}
+  App.draw()
+  local y = Margin_top
+  App.screen.check(y, "I'm ada", 'F - test_click_on_wrapping_line_rendered_from_partway_at_top_of_screen/baseline/screen:2')
+  y = y + Line_height
+  -- click past end of second screen line
+  App.run_after_mouse_click(App.screen.width-2,y-2, 1)
+  -- cursor moves to end of screen line
+  check_eq(Cursor1.line, 1, 'F - test_click_on_wrapping_line_rendered_from_partway_at_top_of_screen/cursor:line')
+  check_eq(Cursor1.pos, 13, 'F - test_click_on_wrapping_line_rendered_from_partway_at_top_of_screen/cursor:pos')
+end
+
 function test_click_past_end_of_wrapping_line()
   io.write('\ntest_click_past_end_of_wrapping_line')
   -- display a wrapping line