There are several issues with the "complete" button in the task list grid view:

1) The function of the button is NOT to complete the task, rather just to open task details panel (see implemenation [1]). For that reason it should be either renamed to "Details" or something similar, or preferably removed (because details panel is opened just by clicking task row in the grid).

2) This button is shown even after user delegates the task (and is therefore not able to complete it by himself). Try the following steps:
1. Create new ad hoc task
2. open details > Assignments and delegate the task to different user.
3. The task remains displayed in the Active Filter for the current user, with "Complete" button displayed , but the button does nothing (now it doesn't even open details panel).

Please check both issues and fix them.

[1] https://github.com/droolsjbpm/jbpm-console-ng/blob/908c3138b4334d8242e72b11b5bef679abed157a/jbpm-console-ng-human-tasks/jbpm-console-ng-human-tasks-client/src/main/java/org/jbpm/console/ng/ht/client/editors/taskslist/grid/TasksListGridViewImpl.java#L451