Fix#3863. Data Validation default operator is `between`. When Excel writes out a data validation item, it may omit the operator. Xlsx reader will therefore initialize operator to null string. Issue indicates that user wants `between` returned for `getOperator`. A more serious problem is that `isValid` method does not handle this situation correctly. Data Validation is changed to set Operator to the default value if an attempt is made to set it to null string.
Change "mixed" declarations to more accurate types in test members; in particular, change those that would be flagged if we were to run Phpstan at level 9 (we currently run level 8). I may or may not follow up with source code (over 700 level-9 problems remain for src), but, as with strict typing, there is no reason to avoid the effort for test members.
It was necessary to update some doc blocks in src to accommodate this change. However, no executable code is touched.
Fix#3687. Worksheet methods insertNewRowBefore and insertNewColumnBefore call ReferenceHelper insertNewBefore. That function fills in "missing" cells with null values. However, for boundaries, it uses getHighestRow and getHighestColumn. It should be sufficient to use getHighestDataRow and getHighestDataColumn. When there is a big gap between getHighest... and getHighestData..., this can result in a big increase in memory usage, and in file space when saving the spreadsheet. New test InsertTest demonstrates the problem by populating a worksheet with cells A1:D5 (so highestDataRow is 5), but also setting row 1000 to invisible (so highestRow is 1000).
The major part of the change is in ReferenceHelper::insertNewBefore, which will now use getHighestData... for its boundaries when filling in the missing cells. Changes of less impact are made to duplicateStylesByColumn and duplicateStylesByRow so that cells which don't yet exist are not created unless the style that will be applied is not the workbook default style.
As for reducing the file size, Writer/Xlsx/Worksheet is changed so that cells whose value is null or null-string and which use the workbook default style are not written to the output spreadsheet. This requires some changes to existing test ReadBlankCellsTest; I don't think the difference should matter to the end-user.
Fix#3847. Person posing the question calls this behavior "pull formulas", and wants equivalent handling to be available in PhpSpreadsheet. I can do this for formulas in a single cell; I can go horizontal or vertical or (unlike Excel) both. Nevertheless, I call this "partial support" because I cannot think how I can do something similar that Excel does, e.g. putting 2 in cell A1 and 4 in Cell A2, selecting them both and using the fill handle to extend those to rows below, so that cell A5 will contain 10.
PR #3852 was a bit strange. Php-cs-fixer complained about a problem that I could not reproduce on my local system, and it didn't really produce sufficient output to correct whatever the problem was. I was able to guess it eventually, but I need to see if there's a way to get it to be more forthcoming.