Only one field can be editable per row

Actually, it seems that the last one that is saved will win. For example if you go an update column C, then column A then column B then the changes from column C and A will be lost and only column B is saved. You can refresh the page, edit column A, refresh, edit column C and then you'll get the desired result.

Without looking at the code I would guess that the original node object is stored and when you edit a field the original node + the change is saved to the database. When a field is edited the "original" that is stored also needs to be updated.

I was able to solve this for nodes with the following:

<?php
function editablefields_form_submit(&$form, &$form_state) {
  $entity = $form_state['entity'];

  // Reload the node from the database
  $entity = node_load($entity->nid, NULL, TRUE);

  // ..... some text truncated

}
?>

This is obviously not the generic way to do it, but by setting $reset = TRUE you always get the latest from the database before saving. It is also rather inefficient, but perhaps necessary.

Should be fixed by b81e145, where I added the name of the field to the form ID.

Actually, it's not fixed.

The problem is that we are storing the entity in the form cache, and resaving the whole entity which overrides the changes you might have done in some other field.

Damien - any comment on the approach in #2? I am sure you would know of a beter way.

I used the approach in #2, with a minor change - this works well for me.

/**
 * Form submit callback: save the field modifications.
 */
function editablefields_form_submit(&$form, &$form_state) {
  $entity = $form_state['entity'];

  // Reload the node from the database
  $entity = node_load($entity->nid, NULL, TRUE);

  entity_form_submit_build_entity($form['#entity_type'], $entity, $form, $form_state);

  // Push the updated entity back into $form_state
  $form_state['entity'] = $entity;

  // TODO: needs a try / catch.
  entity_save($form['#entity_type'], $entity);
}

Fixed in commit e359ace, using a similar approach.

Moving the issues back to the main project.