We can allow horizontal scrolling to potentially avoid this problem.

That is absolutely true. I opened this issue because I couldn't naturally scroll to the later columns, and I cannot remember if/how this can be enabled in Shiny apps.

options = list(scrollX = TRUE) in the DT::datatable should suffice

Can we close this one?

If someone had fixed it, then yes.

Should be fixed now in the branch fede_edits, soon to be pushed.

Example to see how this allows the horizontal scrolling:

## building up on...
example(iSEE,ask = F)
## and slightly editing that `sce` object

rowData(sce) <- DataFrame(primacolonna = sample(1:20908),
                          secondacolonna = paste0("seconda_",sample(1:20908)),
                          terzacolonna = paste0("testobellolungohehe", sample(1:20908)),
                          quartacolonna = paste0("2otestobellolungohehe", sample(1:20908)),
                          quintacolonna = paste0("3otestobellolungohehe", sample(1:20908)))
iSEE(sce)

--> and scrolling on the datatable object, which now also respects the column width specification

My computer at home doesn't have enough grunt to run iSEE (or anything much, for that matter) so I'll assume that scrollX=TRUE does a sensible job. It doesn't scroll if it doesn't need to, right?

Yep, no scroll if not needed. You can check how this is turned on when you resize the window where iSEE runs.