Custom Tree Section, how to toggle checkbox

[npreiser]

I have implemented a custom tree (on the side bar left) and using this tester I am able to select items within the tree, but am unable to check the box on the left of the items. I have tried to "send keys" like spacebar to the item, but it gives me a item not interactable. Any ideas how to check the check box?,

[djelinek]

could you please share the screenshot how the sidebar with checkboxes looks like?