When you test item preprocessing step you get the form with 2 elements:

• value
• value timestamp
• prevous value
• prevous value timestamp

Last 3 are needed only for 2 cases (change and change per second) out of 23 preprocessing steps currently available, still they are displayed everywhere.

1. Go to any item configuration and cilck Preprocessing tab.
2. Click Add, select JavaScript and enter "return value" as parameter.
3. Click Test.

The unused form elements are displayed as disabled as see on the screenshot, which is confusing for a user. I suggest to offer Test form according to the preprocessing step that's being tested:

Suggestion is to get rid of excessive data and make the form human-readable for tests that require 2 values by using a drop-down: