- box-sizing
- flash screen when redraw canvas
reason: execute order
// drwa pic
draw()
// clear pic
clearRect()
draw()
// redraw pic
draw()
solution:
// drwa pic
draw()
// clear pic
clearRect()
// redraw pic
draw()
- unload
页面卸载后，开发工具存在不走onload的问题，但是真机不存在
