If a user opens a very large system file (or an input source with high latency) in the GUI, there is little delay displaying that file.  However, when she scrolls down through the data, there is a noticeable delay whilst the data is read from disk (or other source).

In extreme cases, this can't be avoided, but typically the user will pause, to view the first part of the data, before hitting the scroll down button.  This pause provides an opportunity to preload the data.

To do this we need an additional function to src/data/datasheet.c of the form

bool datasheet_preload_next_chunk (struct datasheet *d, int size);

which loads the next chunk of data and returns true if there's more data to be loaded.

The GUI can then call this function in a low priority idle callback  until it returns false.