REPL sometimes doesn't Post

[telephon]

Irregularly, but frequently, the post window doesn't show the result of the code evaluated.
No reproducer, can be any code.

[telephon]

maybe related to #1217

[telephon]

A reproducer by Stephan Wittwer:
Post without trailing newline is broken, try:

    // okay
fork { 5.do { ".\n".post; 0.5.wait; }};

    // not okay
    // (nothing posted or invisible)
fork { 5.do { ".\t".post; 0.5.wait; }};
fork { 5.do { ".".post; 0.5.wait; }};