The modern player style in Storyline 360 is responsive, which means it adapts to the available screen space. If your player properties window isn’t big enough to preview the entire player, the sidebar will automatically collapse. You can still access the sidebar by clicking the menu icon (☰) on the player.

Here’s how the player properties window looks when the sidebar is collapsed.

You can also make the player properties window bigger to expand the sidebar. Just drag any edge or corner of the window with your mouse.