The moment a file is loaded up it should gain a number- and that is it
If you somehow sort them out, the sort order would hold out- just like it does now whenever you enter anything into Filter box; does it not?
And if you in Mp3tag delete or remove a file temporarily numbered 125- then it is simply removed from that temporary list. And then when you add a new one- then it becomes [e.g.] "file #10001" and not "file #125 but new" [or it could indeed take the now missing number - but that is for the programmer to decide what would work better]
When you go back from using reverse to normal- the filter expression is then either applied once more [which could take time, which would be a downside] or the positive [normal] files are shown from the memory / list- and that choice behavior could be configurable as well [just like in case of Auto-apply option being turned on / off]. That second option would of course show then also the missing files- so it would misinform the user
But such ghost / blank files already are capable of existing in Mp3tag. Here is a proof:
Reverse would always be created when filtering expression would be entered or changed. With 100 000 files it will take time- so some kind of progress bar indicating that would make it possible for the use to see if the reverse is ready to be pushed. Or instead of progress bar a simple grayed out Reverse button turning green [or whatever other color / form]] would tell the same thing
Do you memorize the FILENAMEs of what you drag and drop to Mp3tag in order to be able to filter them later on?
Purpose of number? To id them. So that the Mp3tag would now that file #567 was not shown when filtering expression was entered- which would automatically mean that when the reverse option is activated, file 567 is to be shown. [And to avoid another question] When another expression would be entered, that file would still have this number. And if this time it would show in the normal mode, then it would be not as much as removed from the list- but the list would be created from scratch- is it not what now is happening: you change expression in the Filter box thus you change the outcome shown in the main window. The difference is that now a list [be it for positives or negatives] is not created in a form of file