Add loader between trials

[mrzdcmps]

Hi!
I have some trials that need "on_start"-loading of external ressources. This can take a few seconds. I would like to add a loader to indicate that the next trial is being loaded. I am unsure where to put my code though (render a loader-div's display.style=inherit). I have tried it as content of the new trial , during on_finish of the previous trial and as an onclick event of the previous trial's button. No luck so far.

While the next trial is loading I am seing the previous trial with a "clicked-down button". Anyone has an idea on how to implement a loader?