@Siemienik I remember we had a discussion around cloning/copying style objects - could you review this part of the new code?

I think that the best idea to maintain style's is change logic of setters to use Object.freez and copy values there. ( When any setter style is being used then all style object should be copy and freez)

I didn't know yet how this solution impact to this pr.

For instance:

set fill(fill){
 style = Object.freez({ ...style, fill:Object.freez(fill)});
}

(Written on smartfon - sorry for any mistake)

Edit: this will made styles immutable and protect us

Style setting logic is based on already existing logic in splice method.

rDst.style = rSrc.style; // eslint-disable-next-line no-loop-func rSrc.eachCell({includeEmpty: true}, (cell, colNumber) => { rDst.getCell(colNumber).style = cell.style; }); rDst.height = rSrc.height;

Are you suggesting something like, this:
rDst.style = Object.freeze({...rSrc.style}); // eslint-disable-next-line no-loop-func rSrc.eachCell({includeEmpty: true}, (cell, colNumber) => { rDst.getCell(colNumber).style = Object.freeze({...cell.style}); }); rDst.height = rSrc.height;

@Siemienik are you saying, to do it like this:
rDst.getCell(colNumber).style = Object.freeze({...cell.style}); });

@Siemienik the existing property specific setter actual logic is in "_applyStyle" in row.js and again in cell's setter. This is not using Object.freeze yet. This may be added in another PR.
This pr directly copies style internally, just like spilceRows.
I have update the copy logic with Object.freeze as you told.
@Siemienik @alubbe Please review. Thank you.