a bug I've never run into

The previous commit was failing inside a test that I can't reproduce
manually. Perhaps it's something about how often draw/update run in
practice. Anyways, it's definitely uncovered a real issue.

drawing.lua

@@ -241,9 +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
-    return  -- first draw?
-  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)

drawing_tests.lua

@@ -643,6 +643,7 @@ function test_undo_name_point()
   check_eq(p2.y, 36, 'F - test_undo_name_point/baseline/p2:y')
   check_nil(p2.name, 'F - test_undo_name_point/baseline/p2:name')
   check_eq(#Editor_state.history, 1, 'F - test_undo_name_point/baseline/history:1')
+--?   print('a', Editor_state.lines.current_drawing)
   -- enter 'name' mode without moving the mouse
   edit.run_after_keychord(Editor_state, 'C-n')
   edit.run_after_textinput(Editor_state, 'A')
@@ -650,6 +651,7 @@ function test_undo_name_point()
   check_eq(p2.name, 'A', 'F - test_undo_name_point/baseline')
   check_eq(#Editor_state.history, 3, 'F - test_undo_name_point/baseline/history:2')
   check_eq(Editor_state.next_history, 4, 'F - test_undo_name_point/baseline/next_history')
+--?   print('b', Editor_state.lines.current_drawing)
   -- undo
   edit.run_after_keychord(Editor_state, 'C-z')
   local drawing = Editor_state.lines[1]

edit.lua

@@ -352,7 +352,10 @@ function edit.keychord_pressed(State, chord, key)
       State.cursor1 = deepcopy(src.cursor)
       State.selection1 = deepcopy(src.selection)
       patch(State.lines, event.after, event.before)
-      Text.redraw_all(State)  -- if we're scrolling, reclaim all fragments to avoid memory leaks
+      -- invalidate various cached bits of lines
+      State.lines.current_drawing = nil
+      -- if we're scrolling, reclaim all fragments to avoid memory leaks
+      Text.redraw_all(State)
       schedule_save(State)
     end
   elseif chord == 'C-y' then
@@ -364,7 +367,10 @@ function edit.keychord_pressed(State, chord, key)
       State.cursor1 = deepcopy(src.cursor)
       State.selection1 = deepcopy(src.selection)
       patch(State.lines, event.before, event.after)
-      Text.redraw_all(State)  -- if we're scrolling, reclaim all fragments to avoid memory leaks
+      -- invalidate various cached bits of lines
+      State.lines.current_drawing = nil
+      -- if we're scrolling, reclaim all fragments to avoid memory leaks
+      Text.redraw_all(State)
       schedule_save(State)
     end
   -- clipboard