When the Spreadsheet is rendered, only the cells that are visible are rendered actually. When you scroll, it is a "fake scroll" as the spreadsheet is not actually scrolled but the content of the cells is changed so that it seems that the Spreadsheet is scrolled.
With that said, adding any kind of attachments would be very hard to implement and I am afraid that currently, the Spreadsheet does not support attachments in the cells.
You can submit a feature request in the Kendo UI UserVoice
for the Kendo UI Spreadsheet, which is used by the RadSpreadsheet wrapper internally. There you can include more details on the functionality and sample business cases. This will let the community vote and comment on the idea. Once it becomes popular, the management will prioritize its implementation.
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