How does sync slicers work in a custom visual?

Further details: the example timeline slicer on Github does work. It's a good deal more complex than my slicer, and I can't currently spot what difference might be significant.  To make things even more bizarre, occasionally there is data in the jsonFilter field. Here's my full capabilities.json:

{
    "dataRoles": [
        {
            "displayName": "Date Range",
            "name": "category",
            "kind": "Grouping"
        }
    ],
    "objects": {
        "general": {
            "displayName": "General",
            "properties": {
                "selected": {
                    "type": {
                        "bool": true
                    }
                },
        "filter": {
            "type": {
                "filter": true
            }
        }
    }}
    },
    "dataViewMappings": [
        {
            "conditions": [
                {
                    "Category": {
                        "max": 1
                    }
                }
            ],
            "categorical": {
                "categories": {
                    "for": {
                        "in": "category"
                    },
                    "dataReductionAlgorithm": {
                        "top": {}
                    }
                }
            }
        }
    ],
    "suppressDefaultTitle": true,
    "supportsHighlight": true,
    "supportsSynchronizingFilterState": true
}