The HeaderBar control currently has three areas: leading, center, and trailing. Additionally, there's leftSystemInset and rightSystemInset, which are not RTL adjusted. I've come to the understanding that there is no particularly good reason for this, because every time you would want to use this information for layout purposes, it should also be adjusted for RTL.

With this in mind, the HeaderBar control should be changed as follows:

1. Rename leading to left, and trailing to right, which aligns the terminology with BorderPane.
2. Adjust leftSystemInset and rightSystemInset for RTL.
With this change, the HeaderBar control is more semantically consistent and easier to use, and the renamed left and right areas now show its close relationship with BorderPane.