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.