I would also like this future now that the workaround doesn't work in the latest version

Hello! Thanks for the workaround! works perfectly

I was able to keep the validation as well, but I can't get it to trigger and show the popup error. 
Is there any way I can achieve this? 

thanks!

This is my workaround - it retains the original cells formatting (the formats that we use anyways, not all possible formats are listed)

kendo.spreadsheet.Clipboard.prototype.paste = function () {
                        var sheet = this.workbook.activeSheet();
                        var pasteRef = this.pasteRef();
                        var content = this._content;

                        var currBackground = sheet.range(pasteRef).background();
                        var currColor = sheet.range(pasteRef).color();
                        var currFontSize = sheet.range(pasteRef).fontSize();
                        var currFontFamily = sheet.range(pasteRef).fontFamily();
                        var currTextAlign = sheet.range(pasteRef).textAlign();

                        var currEnable = sheet.range(pasteRef).enable();
                        var currItalic = sheet.range(pasteRef).italic();

                        sheet.range(pasteRef).setState(content, this);
                        sheet.triggerChange({ recalc: true, ref: pasteRef });

                        sheet.range(pasteRef)
                            .background(currBackground)
                            .color(currColor)
                            .bold(true)
                            .enable(currEnable)
                            .italic(currItalic)
                            .fontSize(currFontSize)
                            .fontFamily(currFontFamily)
                            .textAlign(currTextAlign);
                    };

At least give us a workarround...
It is so inconfortabe not being able to use the paste and lost the validations

Absolutelly needed, I have a big problem when I use dates because if I paste on it a date, the validation is gone. User is forced to type on it and dont use the paste, that is not good for user experience, paste only values as excel does should be so helpfull

This would be very highly appreciated. We noticed that pasting data from Excel into Kendo Spreadsheet is extremely slow because Kendo wants to paste the styles too. Pasting e.g. 200 rows and 1 column takes 4-5 seconds, but the more columns you have the longer it takes - exponentially longer. With 3 or more columns it freezes the browser completely. We made a patch in our app that removes the HTML from the pasted data and this makes pasting this amount of data already possible (no hanging anymore), but it is still slow (30 seconds). Further removing Kendo's strange desire to reset row heights for pasted data makes pasting very fast, but we have not found a way to only disable this feature when pasting data.

Allowing devs to control the pasting behaviour would be fantastic.

Excel lets you copy values only via it's paste menu, which will prevent overwriting the validations/format. Kendo Spreadsheet at a minimum needs to give the dev control over paste operations.