Heatmap cell borders disappear on hover (inconsistent with documentation)

[neillrobson]

Expected behaviour

When a heatmap chart's series.heatmap.marker.states.hover.lineColor is left unset/unspecified, the hover state of a given cell should have the same line (border) color as the normal/default state.

I assume this behavior is expected based on the documentation:

• For series.heatmap.marker.states.hover.lineColor: "When undefined, the series' or point's lineColor for normal state is used."
• For series.heatmap.marker.lineColor: "When undefined, the series' or point's color is used."

Actual behaviour

When lineColor for the heatmap chart's hover state is left unspecified, the hovered cells on a heatmap lose their border altogether.

Live demo with steps to reproduce

https://jsfiddle.net/jm7huncp/

• Note that, by default, hovering over the cells causes that cell's border to disappear
• A workaround is to add lineColor explicitly to the hover state (uncomment that line in the code)
• Back before Highcharts 8.1, when using borderColor, the color inheritance across states worked in a manner consistent with the documentation

Product version

Highcharts v10.2.1 (current as of writing)

Affected browser(s)

• Chrome (at least: this is the only browser I tested so far)

[raf18seb]

Hi @neillrobson, thank you for reporting the issue!

lineColor is reset on hover in the pointAttribs method (the stroke is undefined so the color is set to 'none').

Workaround: if no marker states options are found, use stroke for a normal state.
Demo: https://jsfiddle.net/BlackLabel/e0qwLtyu/

[neillrobson]

Ah, that makes a lot of sense!

It looks like, based on your demo JSFiddle, the relevant line of code in the library that needs to be updated is here. Does that look correct to you?

If so, I may have the bandwidth in the next few days to create a pull request with this fix (alongside unit test updates & whatever else may be necessary). Let me know if that would be helpful and I can try to make time for contributing.