There is a double arrow (↔) next to the packet explorer. If you cancel the check, Eclipse will not focus the Explorer Explorer stack when opening the resource.
You can also choose what to show in the package explorer. In the package explorer menu (top right, next to the double arrow), select Filters ... If you disconnect (select) Libraries from the external, then the package explorer will not display libraries (and therefore will not expand them). However, this means that no libraries will be shown (not even with the source). This is not exactly what you want, but it can help.
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