Header plugin actions: theme data reference

Reading the data tables. Presence varies by page and state; this is not a requirement to populate every field.

Field	Type	Meaning	Availability
viewModel.headerActionContributions	array	Prepared header action contributions collection; may be empty.	When the parent is provided; check optional fields.
viewModel.headerActionContributions[].html	string	Prepared HTML; preserve the supplied markup.	For each existing item; optional child fields depend on item type.
viewModel.headerActionContributions[].id	string	Id value supplied for this context.	For each existing item; optional child fields depend on item type.
viewModel.headerActionContributions[].placement	string	Placement value supplied for this context.	For each existing item; optional child fields depend on item type.
viewModel.headerActionContributions[].sortOrder	number	Sort order numeric value.	For each existing item; optional child fields depend on item type.