This isn't really a bug, but you are running into a convenience feature.

When the focus rectangle isn't blue, that means that the plugin list is actually not focussed. In this case, normally you wouldn't even be able to move the selection with the cursor keys inside this control. However, in order to be able to operate the plugin list quickly even when the Filter text field is focussed, pressing the cursor keys also works when the plugin list doesn't have focus. But in that case, as the intention is to use keyboard navigation in conjunction with the filter, folder items are automatically skipped (as you want to find a specific plugin, not a folder). This is however not done when the plugin list is actually focussed, you can navigate to folders in that case.

Oh, I allready figured it out. It can get focus by pressing Tab a few times. I was to quick in reporting this. Sorry!

I guess there's one thing we could do to make this a bit more obvious: Change the tab order so that the plugin list comes directly after the filter text field. Since the "Put in FX..." button can already be triggered by the Return key, and the Cancel button by pressing the ESC key, there is no need for them to be so high up in the tab chain.

That would be great if this is easy to implement. For me, what I really like about Openmpt is that you can almost completely get by with keyboard commands. So anything that smoothens this keyboard driven workflow gets my vote!

Tab order was improved in r18861, also tab focus was removed from the two locked edit fields that contain the path and plugin vendor, improving keyboard navigation even more.