When the TitledPane is expanding we can see the content briefly before the background is visible. When the TitledPane is collapsing the content is briefly visible after the background is gone.

This is visible after fixing RT-20225

The cause is when the content is at its min height it is centered to the title region and slightly extends below the titled region. See attached images with clipping turned off. The fix is to introduce a container for the content and set its alignment to be Pos.BOTTOM_CENTER