After you select a message and it is displayed in the preview pane you can still select other messages but there is no option to delete all the selected messages while previewing the current message. (A picture is worth a thousand words but there is no easy way to insert an image - redacted of course - into the post.)
Would you like the preview pane to be closed when selecting other messages, perhaps?
This is still the case.
I think I would prefer the actions (move, delete…) to always appear, no matter if there is a preview or not.
However if your solution is easier to implement I guess it makes sense as well.
If I’m selecting several messages is because I want to do something with them (move, delete…), the preview is secondary at that point I guess.