New Windows Appear on Panel Out of Order

[tek10]

I think the setting you need to change is the sorting order for the panel window buttons.

Menu
  Settings
      Panel
        Items
Click once on Window Buttons to highlight.
Click the button to edit the currently selected item.
Look for Sorting Order and in the drop down menu select Timestamp.
Close the settings windows and I believe the items will then open and display from left to right as you want.