When setting the selectable flag on a GtkLabel the text don't automatically wrap anymore. Happens at least for macOS.
Issue happened on this meanwhile reverted patch https://git.libreoffice.org/core/+/729ba6b6f79884cdd4a24bc9326a06a2c319fc04%5E%21
The real GtkLabel works find, in the non-native gtk case this selectable Label is mapped to SelectableFixedText which inherits from Edit and pretends to be a label. That would have to be changed to support this. e.g. inherit from VclMultiLineEdit
See also bug 128524 comment 10.
No benefit in keeping this ticket open. All issues caused by the fact seems to be solved.