On a table, the user should be able to edit the values in each cell on a specific column. The current solution is using a multi selection drop-down menu but the user has to scroll through the options, which is a bit inconvenient. The table is wide, and inserting a button into the cells is a problem. adding a contextual button above the table is an option, but that means that the contextual menu has to be fixed, and and it might be a problem since the menu above it isn't fixed. once the button is triggered a pop up window with the cell setting will appear.
Can you recommend a conventional method to enable edit of a single cell?
Perhaps you could pop up the allowable options in a box as the user types them? This sort of auto-complete is similar in functionality to the tags editor that you used to add tags to your question.
If you're concerned that the user might not know what rules are available, you could always provide a filter-by-typing approach, accompanied by a detailed reference list. What I mean is, you start by showing every rule in a tabular format (uses space most efficiently) and filter down the list of rules as the user types, eventually showing "No rule found" if they've typed something you can't find a match for. Then, the user would go to the reference list to locate whatever they're needing.
The benefits to this system is that, after the first time they use a rule, the user will be able to very quickly fill it in at another position. I would argue that this beats manually navigating a list every time (even with a recently used section)
As per my view point, Use inline edit to edit single cell and if column contains multiple fields to select like a dropdown checkbox then show respective controls over the cell when user click the cell to edit. For an example : If column is for age then show calender on edit, if it is either or option then show dropdown.