Theme overrides are not applied with missing optional base values

[nickofthyme]

Describe the bug
When using a theme override in certain circumstances the first values do NOT take precedence and revert to the baseTheme value.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/happy-violet-xcb3z?file=/src/App.tsx

Expected behaviour
In the example codesandbox above, I expect the crosshair value to be applied from theme2 but it is actually being applied from the baseTheme.

Additional context
Likely related to the mergeOptionalPartialValues option on the mergePartial utility function.

Related to https://github.com/elastic/kibana/pull/106845/files/039ca4ef1a815cc9d76c9ebbad3ce7a00be26115..ac3eb6a10fbe552743d7a9075c104a2c010ea043

[rshen91]

@nickofthyme I think I'm following. The issue I'm facing is that I'm not able to get this theme override to work without setting up the tooltip prop in the <Settings />. I kind of used the Grid lines.story.tsx as an example, but I wasn't able to get the crosshair cursor to work at all without setting tooltip={getTooltipTypeKnob('Tooltip type', TooltipType.Crosshairs)}

https://codesandbox.io/embed/ancient-leftpad-9klp9?fontsize=14&hidenavigation=1&theme=dark

[rshen91]

I think in kibana to get the cursor to be crosshairs changing the TooltipProps to the following is showing:

const tooltipProps: TooltipProps = tooltip.detailedTooltip
    ? {
        ...tooltip,
        type: TooltipType.Crosshairs,
        boundary,
        customTooltip: tooltip.detailedTooltip(headerFormatter),
        headerFormatter: undefined,
      }
    : { ...tooltip, boundary, headerFormatter };

[rshen91]

Thanks for the zoom to clarify @nickofthyme 👍🏻

[nickofthyme]

Fixed in #1410 by making mergeOptionalPartials true by default.

[nickofthyme]

🎉 This issue has been resolved in version 38.1.4 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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