OK, I've tested with your library, and here what happened:
- At first I was not able to replicate the problem.
- I upgraded my calibre version from 7.7.0 to 7.12.0. After the upgrade I tested again and was able to replicate the issue on the new version. To pinpoint what is the earliest calibre version that causes this issue, I tested multiple versions. It turns out the issue starts with calibre version 7.8.0
- The problem is unrelated to test version of the plugin. It happens with previous versions of the plugin as well.
- The issue happens because Modifier Selection validates all saved searches before opening the dialog. It works perfectly on calibre version older than 7.8.0, because the plugin makes sure to add restriction of one book when validating the saved search.
- There is modification in calibre starting in version 7.8.0 that seemingly makes the one book restriction not work anymore.
- As a result, the dialog takes a long time to open on recent calibre versions until it completes validating all saved searches. But if you give it enough time, it will ultimately open.
- I can fix this at my end by only validating the saved search that the user will ultimately choose from the drop menu. This will not completely solve the issue, but will significantly mitigate the issue on most libraries. But I've seen users whose libraries have hundreds of thousands of books. Validating even one complicated search without being able to set the restriction might take some time.
- I also will have to turn off the validate as you type for regular searches, because they are also affected by this.
- I tried to pinpoint the commit in calibre base that made this change to no avail.
I will post a test version shortly.