Openpyxl can generate the xml tag `<patternFill/>`, possibly even as a default style. Excel has no problem with this, treating it as "fill none", but PhpSpreadsheet has a glitch because it treats it as "fill solid white". So, when PhpSpreadsheet loads and saves such a file, the result at first appears as if gridlines are disabled; in fact, the gridlines are merely invisible behind the cells with their solid white fill. This PR makes PhpSpreadsheet behave the same as Excel in this circumstance.
Co-authored-by: Mark Baker <mark@lange.demon.co.uk>
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