For bulk edit: even if stock status was left as "No change", the
status of all variations was being changed to whatever the status
of the product was before it was converted to variable. Now
no change is performed when "No change" is selected, and all
variations change to whatever is selected otherwise.
For quick edit: a new "No change" option is added that will be
preselected when the product is variable. Previously, whatever
status the product had before being converted to variable was being
shown, and that's the status that would be set when saving.
Also, a "This will change the stock status of all variations"
message is displayed before the selector.
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