Pause untill Custom Floating HTML Prompt window closes

I'm trying to let KB Maestro know when a Custom Floating HTML Prompt window has been closed.

Here's what I've tried so far:

I've given the Custom Floating HTML Prompt window a custom title, named 'customwindowtitle' by inserting this line in the HTML text:

<title>customwindowtitle</title>

That works; the custom title is visible in the window bar. My next step would be to let KB Maestro pause until the window closes. Normally I would do this with inserting the "Pause Until Conditions are Met" action with the following settings:

Pause Until Conditions are Met:
Any window title of:
Keyboard Maestro Engine
does not contain "customwindowtitle"


This doesn't work. It also seems that the Custom Floating HTML Window title is not listed through this method: SCRIPT: Get List of All Window Names of "Windowed" Apps using JXA (thanks @JMichaelTX!)

Any (other) ideas on letting KB Maestro 'know' when a Custom Floating HTML Prompt window has been closed?