bugfix

As usual, binary search is hard to get right. This time I was never
actually selecting between left and right when they were just one
character apart.

manual_tests

@@ -2,6 +2,7 @@ file load:
   cursor_line = 1
   first line is a drawing -> cursor_line = 2
 click on text -> cursor moves
+click on first character of text -> cursor on first character of text
 click on drawing -> cursor doesn't move
 create drawing -> cursor bumps down below drawing
 backspace

text.lua

@@ -298,20 +298,25 @@ function Text.nearest_cursor_pos(line, x, hint)
       left = hint
     end
   end
-  while left < right-1 do
+--?   print('--')
+  while true do
     local curr = math.floor((left+right)/2)
     local currxmin = Text.cursor_x(line, curr)
     local currxmax = Text.cursor_x(line, curr+1)
+--?     print(x, left, right, curr, currxmin, currxmax)
     if currxmin <= x and x < currxmax then
       return curr
     end
+    if left >= right-1 then
+      return right
+    end
     if currxmin > x then
       right = curr
     else
       left = curr
     end
   end
-  return right
+  assert(false)
 end
 
 function Text.cursor_x(line_data, cursor_pos)