Race condition can cause JS to reference nonexistent elements

[speezepearson]

Consider this code:

import time
from browsergui import Button, GUI
b = Button()
@b.def_callback
def _():
  b.text = "Clicked"
  gui.body.remove(b)
  # browser requests command
  gui.body.append(b)

gui = GUI(b)
gui.run()

If the browser requests a lump of JS at the marked point in the code, the generated JS can look like "wipe out everything in the document body; then get the button element and..."; but the button element no longer exists, so executing the JS raises an error in the browser, which ends up closing the browser window. (This won't always happen, because the order of the JS commands is nondeterministic.)

I think DocumentChangeTracker needs a serious refactor. It's always felt a little wonky to me, and the fact that this kind of error is possible (even though I tried to threadsafe it) suggests something is horribly wrong with the architecture.