[system] Fix theme mutation when using responsive typography shorthand in sx#48266
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…pography shorthand in sx The responsive branch of setThemeValue assigns `css[mediaKey] = finalValue` where finalValue is a reference to a theme.typography variant object returned by getPath. This replaces the breakpoint's CSS accumulator with the live theme object, so subsequent responsive sx keys targeting the same breakpoint write their CSS properties directly into theme.typography. Use merge() instead, consistent with the non-responsive path on the next line.
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mj12albert
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mj12albert
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Apr 13, 2026
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Thanks for the PR @tomups |
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Summary
Fixes #48265
setThemeValueinstyleFunctionSx.jsassignscss[mediaKey] = finalValuewherefinalValueis a theme typography variant object returned by reference fromgetPath. This replaces the breakpoint's CSS accumulator with the live theme object, so subsequent responsive sx keys targeting the same breakpoint write their CSS properties directly intotheme.typography.<variant>.For example, after rendering
<Box sx={{ typography: { md: 'h4' }, width: { md: '80%' } }}>,theme.typography.h4permanently containswidth: '80%', affecting every component using that variant.The non-responsive path already uses
merge(css, finalValue)which copies properties without creating a reference. This PR makes the responsive path consistent by usingmerge(css[mediaKey], finalValue)instead of the direct assignment.Regression from #44254.