Evaluation order of Button[]'s action

Question

it looks like it should be well known issue but I haven't found answer anywhere. Please tell me why the result (after clicking) of this:

Button["X", Print@1; Pause@1; Print@2;]

is different from result of this:

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

Answer 1

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.

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"