Note: this message composed before post #16 was posted
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DaltonST - Suggestion - add another check box modifier "Multiline?"
If unchecked It would be entered/edited via a regular text box either in the MDE or the book list, and it would be capable of being explicitly 'positioned' in Book Details - e.g. if was Subtitle one would probably position it immediately after Title.
If it was checked it would require a simple multi line text editor, Qt surely has one.
I appreciate this somewhat beyond the scope of what you're actually doing: adding a label to long text columns. So, "That's beyond the scope of what I'm trying to do" would be an understandable response
For me, it is essential I can to continue enter/edit data into 'long text' columns from the book list by pressing F2 and using the free standing 'Comments' Editor. I only mention it because I know my usage pattern, which includes avoiding the single MDE dialogue like the plague, deviates from the norm

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In Book Details I would not like to see the content of long text columns aligned according to the length of the longest label, as happens, quite properly, with other column types. Actually, I would rather see the label shown on a separate line, e.g.
Label: and the text shown full width as it is now.
In Book Details, will you address the issue that the content of a custom long text column is not demarcated from the regular Comments text. I overcome this by having a separator line at the bottom of my custom long text column template.
You ask -
Why would you ever not use a label? In my case probably because I have 47,363 books that have existing quasi-labels as previously described in post #9, and that I'm habituated to pasting a template either from my snippets manager or from Evernote
BR