"When clause" in past tense when describing a conditional fact

Please take a look at the following example of using the When conjunction:

We’ve fixed a bug where an error message popped up when the user clicked / clicks on the Start button.

If the main clause takes a past form (popped up) as in the example above, which is correct between "clicked" and "clicks"? I know that typically the subordinate clause also should take the same past tense; however, this sentence describes a conditional fact, rather than indicating an actual incident that ever occurred in timespace. Does this rule still work in this case?