[racket] Showing a hidden frame from X11

Is there a simple way to show a frame/window (that previously hid itself)
from the outside of the process?

I created a simple debugging dialog window for this purpose:
https://gist.github.com/Metaxal/bab50f9cd6407b9a02cd

Here's what I want to do:
1) Run the file, the dialog shows up with a button on the middle.
2) Click the button, this hides the frame by calling `(send frame show #f)`.
3) From the outside of the process, re-show the frame using X11 calls.

I tried to call X11's show, map, raise, set-input-focus, but nothing of
that works: none of these events are received by the dialog, and no message
is printed at all.

Calling X11 show method actually spawns an empty black window, as if the
frame had not received the message.

Note that at the end of the file there is a `(yield never-evt)` to prevent
the main thread from exiting. Maybe this has a nasty side effect with the
GUI thread?
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