Evaluation order of Button's action

It looks like a basic issue but I haven't found an answer anywhere.

Please tell me why the behavior (after clicking) of this Button:

Button["X", Print@1; Pause@1; Print@2;]
    (*1 and 2 appear simultaneously after 1 sec*)

is different from the behavior of Shift+Enter evaluation:

Print@1; Pause@1; Print@2;
    (*printing of 2 happens 1sec after 1*)

Answer

The reason is because Button actions are calculated on a preemptive link, meaning they preempt any other evaluation, but are only allowed a certain amount of time to evaluate. That indicates:

the front end sends one evaluation at a time and waits for the result before continuing with its other work

So it can't display 1 because it is waiting for Pause[1];Print[2] to finish.

You can replicate the behavior of

Print@1; Pause@1; Print@2;

by adding the option Method->"Queued" to the Button arguments.

This ensures the actions are performed in the current queue and no time limit is enforced.

See the documentation under "Details and Options"

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Further reading: