In T298259: Add button to sidebar to hide/show unused fields we implemented a "hide unused parameters" filter for large lists of template parameters. This is a follow-up to that.
- Whenever the dialog opens with a single template, unused parameters are shown.
- When a multi-part transclusion is edited, all unused parameters in all parts are initially hidden.
- All this must consider that the filter options are hidden for templates with 3 or less parameters. Contrary to what was said above unused parameters are not hidden when a template has 3 or less parameters.
- When adding more parameters to a template so it includes more than 3, the search and hide unused widgets will show, but unused parameters should (still) not be hidden
- When adding another part to a multi-part transclusion, the new part should never be collapsed.
- This allows for more performance improvements. See T300974.