push action registers the action to be executed by the mainloop at the start of the next frame, or at a subsequent frame if the queue is already large.

Warning: the action may also call push itself, but since there is only one execution queue, the second action will be executed only after the first action terminates. For instance this program:

Sync.push (fun () ->
    print_endline "push 1";
    Sync.push (fun () ->
        print_endline "push 2";
        print_endline "end 2");
    print_endline "end 1");

print_endline "Creating board";

W.label "Checking sync... see console"
|> L.resident
|> Main.of_layout
|> Main.run

will print:

Creating board
push 1
end 1
push 2
end 2