Once the user manages to Link External Data (a non-trivial task, see bug 114994, bug 114995, bug 114996, and finally bug 100993), there is no way to tell what ranges of the sheet have imported data.
This means the user can inadvertently edit data in a cell that's part of an imported range, and their data will be lost when the link is updated.
Steps to Reproduce:
1. Sheet -> Link to External Data
2. Paste the URL of an HTML file with table(s)
3. Import the data
Externally-linked data looks just like verbatim data in the sheet.
There should be a visual indication around the imported data, to highlight that it is subject to change.
User Profile Reset: No
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36
Repro with Edit - Links to external.. - Update
attachment 139782 [details] can be used for quick testing
Let's ask UX
Better would be to add some combination of URI and table/range name (polished and de-duplicated) to the defined database ranges, with the benefit that the data range would be selectable and addressable as usual. The tricky part would be to "link the link" for storage and adjust the resulting database range on each update of the linked data, which can happen even time based, but there's already code for updating the range for other database sources.
Future features then could associate a (real) table style with such defined database ranges.
(In reply to Eike Rathke from comment #2)
> Better would be to add some combination of URI and table/range name
> (polished and de-duplicated) to the defined database ranges, with the
> benefit that the data range would be selectable and addressable as usual.
Absolutely, today you don't even see the difference between direct input and what comes from an external source when editing the field. And when the user edits the field manually this change is overridden in case of automatic update. Would expect Edit > Link(s) to External Data to open when I press F2 but only for cells with those links and not when cells are empty.
We could lock these cells somehow as a first step. But additionally it would be nice to see (temporarily) the state of cells. Similar request was made in bug 115241 about conditional formatting. We could show some indicator for the various options like Excel's triangle but with a nicer icon.