Pasting a single line into the custom filter input field shouldn't add it as a filter
We supporting copy-pasting multiple filters into the custom filter input field in the options page which causes them to be automatically added to the list. However, the same behavior applies when pasting in only a single line. Since that single line may not contain a complete filter, we should not automatically add it as a custom filter.
For example, a user may be copy-pasting a domain name, a CSS selector or (parts of) a different filter into the input field to then use it as a basis for creating a filter from it.
What to change
When pasting a single line into the custom filter input field, don't clear the input field and don't add the line as a custom filter.