Story: If a user adds invalid input in good faith we should inform the user immediately that the input can't be used and offer a solution the resolve the problem so that the user can finish what he/she wanted to do.
- Indicate that input is invalid on the input field via inline validation. If possible do the validation on input. If that takes too much resources, defer by n seconds or do it when the cursor leaves the input field.
- Offer a way to call up information about the nature of the problem and how to resolve it
- If a save button is used for several input fields (thus it may not be clear, what causes it not to save or to be inactive), the save button itself should also offer the above possibilities.
- The input field gets a thin red outline. In the input field, an error indicator appears. Also, the save button indicates that saving will not work.
- Same if a field of a qualifier or reference is not valid.
- If the user hovers or clicks the indicator, the error message is shown
- If the user clicks the indicator on the save button an error message is shown, too (we do not need to show them at the same time as shown here for demonstration purposes)
- If the user folds a group of entry fields (thus hiding them) there needs to be an indicator that shows that there is a problem in this group of fields.
- In the future we may or may not stop to hide the message behind an indicator icon. That would look like in the image below. We should not make it too hard to switch to such an solution.