a more radical attempt at ignoring nil y's

app.lua

@@ -35,9 +35,7 @@ function love.run()
       end
     end
 
-    if love.timer then dt = love.timer.step() end
-
-    if App.update then App.update(dt) end -- will pass 0 if love.timer is disabled
+    -- perform draw before update to give it a chance to mutate state
 
     if love.graphics and love.graphics.isActive() then
       love.graphics.origin()
@@ -48,6 +46,10 @@ function love.run()
       love.graphics.present()
     end
 
+    if love.timer then dt = love.timer.step() end
+
+    if App.update then App.update(dt) end -- will pass 0 if love.timer is disabled
+
     if love.timer then love.timer.sleep(0.001) end
   end
 end

drawing.lua

@@ -241,10 +241,6 @@ function Drawing.update(State)
   if State.lines.current_drawing == nil then return end
   local drawing = State.lines.current_drawing
   assert(drawing.mode == 'drawing')
-  if drawing.y == nil then
-    -- something invalidated all the y's; just wait for a draw
-    return
-  end
   local pmx, pmy = App.mouse_x(), App.mouse_y()
   local mx = Drawing.coord(pmx-State.left, State.width)
   local my = Drawing.coord(pmy-drawing.y, State.width)