I have seen something similar in 10.15: Designer will lose focus and I will need to switch back to the window.
But your issue sounds like it may be related to this Designer fix ESB_10.15_Fix4:
At times, Designer 10.15 does not display pipeline contents instantly.
During flow service development or debugging, Designer displays the pipeline
contents for an element with a slight delay of one second or less. Typically,
the delay occurs when the pipeline has multiple document references and
transformers in the mappings.
This issue is now resolved. "
I don’t know if the free trial version includes fixes
#Service-Designer#webMethods