Only sometimes, but sometimes I accidentally delete files from wrong lister. So it would be fine to place the dialog on the side of the lister which is affected by deletion (maybe as general option for commands and not by default).
As always not important, but maybe helpful.
Using the option to highlight the destination side in a different color might help avoid those situations.
Guess. But on 21:9 there're real wide listers and mouse-ways. And using too much different colors (background) is not eye-friendly, while file-display-toolbar is very small.