When I switch on online spell checking in a Text document, next time I create or
open a Spreadsheet document, the option is checked there, too. Now when
switching it off there, next time I open my original Text document, it's
switched off there, too.

This is weird.

Ideally, online spell checking would be a per-document option (where perhaps the
last known setting for a given document type is applied to a new version of this
type). This is because whether or not I want to do spell checking on a document
heavily depends on the concrete document and its content.

Less ideally, but still better than the current situation, would be if the
setting would be per document type. That is, if I switch it on or off in a Text
document, this applies to *all* Text documents, but *not* to other document
types. Reasoning here is that Text documents have a much higher probability of
requiring spell checking than Spreadsheets or even Drawings.