> There are many race conditions that must be dealt with in input and window management because of the asynchronous nature of event handling. [...] However, not all race conditions have acceptable solutions within the current X design. For a general solution it must be possible for the manager to synchronize operations explicitly with event processing in the server. For example, a manager might specify that, at the press of a button, event processing in the server should cease until an explicit acknowledgment is received from the manager.

[1] https://web.mit.edu/6.033/2006/wwwdocs/papers/protected/xwin...