I've been experiencing this small issue ever since I began alpha testing Picard 3 but I haven't been able to narrow down the bug until today.

In an apparent recent regression, it is no longer possible to paste text into a tag when multiple tracks are selected and the field is selected but not in an editable state. This was doable in Picard 2 up until the latest stable version as far as I'm aware.

The problem seems to stem from this method in picard/ui/metadatabox/{}{}init__.py:
    def _can_paste(self):
        mimedata = self.tagger.clipboard().mimeData()
        has_valid_mime_data = any(self.mimedata_helper.decode_funcs(mimedata))
        return has_valid_mime_data and len(self.tracks) <= 1 and len(self.files) <= 1

Changing the return line to the following (provided by an LLM) resolves my concern, although I haven't opened a PR because I don't know what ramifications it carries.
        return has_valid_mime_data and self.tag_diff is not None and self.currentItem() is not None

It's possible this change was intended but I find the old behaviour more intuitive personally, even despite using the alphas heavily for a few weeks now.