keep one screen line of overlap on pagedown

I'm now extracting the concern of computing
line.screen_line_starting_pos out of Text.draw. Earlier
I had to make sure I ran through the whole line to compute
screen_line_starting_pos, but that had the side-effect of updating
Screen_bottom1.pos as well with lines that had never been rendered.

In this process I hit my first bug due to an accidental global. It
doesn't show up in the patch because I accidentally deleted a local
declaration. (I thought I didn't need screen_line_starting_pos anymore,
deleted everywhere, then brought it back everywhere from the bottom of
the function up, but forgot to put back the very first occurrence.)

The amount of yoyoing this caused between App.draw and Text.draw, I very
much have spaghetti on my hands.

Accidental globals are _terrible_ in a program with tests. Cross test
contamination X-(

main.lua

@@ -235,7 +235,7 @@ function App.keychord_pressed(chord)
   elseif chord == 'pagedown' then
     Screen_top1.line = Screen_bottom1.line
     Screen_top1.pos = Screen_bottom1.pos
---?     print('setting top to', Screen_top1.line)
+--?     print('setting top to', Screen_top1.line, Screen_top1.pos)
     Cursor1.line = Screen_top1.line
     Cursor1.pos = Screen_top1.pos
     Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()

text.lua

@@ -19,39 +19,32 @@ function Text.draw(line, line_width, line_index)
   if line.fragments == nil then
     Text.compute_fragments(line, line_width)
   end
-  line.screen_line_starting_pos = nil  -- TODO: avoid recomputing on every repaint
+  if line.screen_line_starting_pos == nil then
+    Text.populate_screen_line_starting_pos(line_index)
+  end
   if Debug_new_render then print('--') end
   for _, f in ipairs(line.fragments) do
     local frag, frag_text = f.data, f.text
     -- render fragment
     local frag_width = math.floor(App.width(frag_text)*Zoom)
     local s=tostring
---?     print('('..s(x)..','..s(y)..') '..frag..'('..s(frag_width)..' vs '..s(line_width)..') '..s(line_index)..' vs '..s(Screen_top1.line)..'; '..s(pos)..' vs '..s(Screen_top1.pos))
+--?     print('('..s(x)..','..s(y)..') '..frag..'('..s(frag_width)..' vs '..s(line_width)..') '..s(line_index)..' vs '..s(Screen_top1.line)..'; '..s(pos)..' vs '..s(Screen_top1.pos)..'; bottom: '..s(Screen_bottom1.line)..'/'..s(Screen_bottom1.pos))
     if x + frag_width > line_width then
       assert(x > 25)  -- no overfull lines
       -- update y only after drawing the first screen line of screen top
       if line_index > Screen_top1.line or (line_index == Screen_top1.line and pos > Screen_top1.pos) then
         y = y + math.floor(15*Zoom)
---?         print('text: new screen line', y, App.screen.height, screen_line_starting_pos)
+        if y + math.floor(15*Zoom) > App.screen.height then
+--?           print('b', y, App.screen.height, '=>', screen_line_starting_pos)
+          return y, screen_line_starting_pos
+        end
         screen_line_starting_pos = pos
+--?         print('text: new screen line', y, App.screen.height, screen_line_starting_pos)
         if Debug_new_render then print('y', y) end
       end
       x = 25
-      if line.screen_line_starting_pos == nil then
-        line.screen_line_starting_pos = {1, pos}
-      else
-        table.insert(line.screen_line_starting_pos, pos)
-      end
-      -- if we updated y, check if we're done with the screen
-      if line_index > Screen_top1.line or (line_index == Screen_top1.line and pos > Screen_top1.pos) then
---?         print('a')
-        if y + math.floor(15*Zoom) > App.screen.height then
---?           print('b', y, App.screen.height)
-          return y, screen_line_starting_pos
-        end
-      end
     end
-    if Debug_new_render then print('checking to draw', pos, Screen_top1.pos) end
+--?     print('checking to draw', pos, Screen_top1.pos)
     -- don't draw text above screen top
     if line_index > Screen_top1.line or (line_index == Screen_top1.line and pos >= Screen_top1.pos) then
       if Debug_new_render then print('drawing '..frag) end
@@ -148,7 +141,6 @@ function test_pagedown_skips_drawings()
   Screen_bottom1 = {}
   Zoom = 1
   local screen_top_margin = 15  -- pixels
-  local text height = 15
   local drawing_height = 20 + App.screen.width / 2  -- default
   -- initially the screen displays the first line and the drawing
   -- 15px margin + 15px line1 + 10px margin + 25px drawing + 10px margin = 75px < screen height 80px
@@ -164,6 +156,39 @@ function test_pagedown_skips_drawings()
   App.screen.check(y, 'def', 'F - test_pagedown_skips_drawings/screen:1')
 end
 
+function test_pagedown_shows_one_screen_line_in_common()
+  io.write('\ntest_pagedown_shows_one_screen_line_in_common')
+  -- some lines of text with a drawing intermixed
+  App.screen.init{width=50, height=60}
+  Lines = load_array{'abc', 'def ghi jkl', 'mno'}
+  Line_width = App.screen.width
+  Cursor1 = {line=1, pos=1}
+  Screen_top1 = {line=1, pos=1}
+  Screen_bottom1 = {}
+  Zoom = 1
+  local screen_top_margin = 15  -- pixels
+  local line_height = math.floor(15*Zoom)  -- pixels
+  App.draw()
+  local y = screen_top_margin
+  App.screen.check(y, 'abc', 'F - test_pagedown_shows_one_screen_line_in_common/baseline/screen:1')
+  y = y + line_height
+  App.screen.check(y, 'def ', 'F - test_pagedown_shows_one_screen_line_in_common/baseline/screen:2')
+  y = y + line_height
+  App.screen.check(y, 'ghi ', 'F - test_pagedown_shows_one_screen_line_in_common/baseline/screen:3')
+  -- after pagedown the bottom screen line becomes the top
+  App.run_after_keychord('pagedown')
+  check_eq(Screen_top1.line, 2, 'F - test_pagedown_shows_one_screen_line_in_common/screen_top:line')
+  check_eq(Screen_top1.pos, 5, 'F - test_pagedown_shows_one_screen_line_in_common/screen_top:pos')
+  check_eq(Cursor1.line, 2, 'F - test_pagedown_shows_one_screen_line_in_common/cursor:line')
+  check_eq(Cursor1.pos, 5, 'F - test_pagedown_shows_one_screen_line_in_common/cursor:pos')
+  y = screen_top_margin
+  App.screen.check(y, 'ghi ', 'F - test_pagedown_shows_one_screen_line_in_common/screen:1')
+  y = y + line_height
+  App.screen.check(y, 'jkl', 'F - test_pagedown_shows_one_screen_line_in_common/screen:2')
+  y = y + line_height
+  App.screen.check(y, 'mn', 'F - test_pagedown_shows_one_screen_line_in_common/screen:3')
+end
+
 function test_down_arrow_moves_cursor()
   io.write('\ntest_down_arrow_moves_cursor')
   App.screen.init{width=120, height=60}