There are a few things to consider. But first off, just as a hint, having a screen shot of the situation would very helpful.
To consider:
- What is the user going to be putting in the combo box?
- Is what the user typing in the combo box dependent on what's in the table?
- Is there a way that you can place the combo box in such a location such that when the options show, you are not hiding any needed content in the table?
There are times and places where sacrifices need to be made, and you need to hide content (but those situations are far and few in-between, and should be avoided as much as possible). But let's look at what other sites are doing. Take Google for example:
If we are typing something into the search box, we are given a drop down of some autocompleted search queries. This drop down does in fact hide content, but the hidden content is navigation, and not pertinent to the activity taking place (the act of searching content).
Now, when it comes to your table, we need to discover if the combo box is going to overlay information pertinent to the action taking place. That is to say, if you are searching a table of products, and you start typing in Cheese, you could get an autocomplete of Cheesey Squeezy, Cheese Puffs, and Kraft Cheese, that would overlay the table. If we have an autocomplete overlaying information we are looking for, then we must find a new place or new way of displaying the autocomplete.
However, if your autocomplete is something more across the lines of an "Filter By" option, then you might actually be able to get away with it. Again, see a Google example.
When filtering our results by time, we are not actively looking at the content, so it is acceptable to cover up the search results.
Now, getting into possible solutions if your autocomplete does in fact cover important content. There are a few options:
- One of the more popular options I've seen is having the autocomplete input on the right side above the table.
Generally, important information in a table is on the left (in the English speaking world, where we most commonly look left to right). So it would be less of a UI hit if we hid less important content on the right side of the table.
The first place that I see this kind of setup is with the Safari web browser:
When the user clicks the magnifying glass, you get two options for searching the page. In this case, the search is on the right, because that is where it is most generally out of the way, allowing a drop down to cover up (hopefully) less important content.
--Added--
- You might also consider putting the drop down inside the list.
This is a less used approach, but I've seen it around. Depending on the size of the list (how large it will become), it might be acceptable to have the Plus button inside the list at the bottom. From the screen shots you added, you essentially have a list (not a table, but that's semantics), so you could make the Plus simply the last entry in the list, and as the list grows, the Plus would always be at the bottom. This gives a Todo List type feel to the table, and that may or may not be appropriate. A benefit from this is that the dropdown will never cover up content inside the list, because it is always at the bottom. And since the Plus is still inside the table, the control isn't as disjointed as it would be if it were outside and below the list.
- Another option would be to make the drop down for autocomplete actually "drop up."
This is a less used approach, and depending on location of your table and autocomplete, may not be a viable option.
It is typically not a good approach to put an input box below the area it will be used. You are right that it will have lower discoverability by placing it below the table. In web world, users generally read (I say generally because the web is filled with strangeness) top to bottom. Having an input below the table would cause a disjoint, and most likely, users will think that the input goes to some element farther down.
Edit:
Added screen shot of Safari Web Browser for use case with drop down on the right side of content. Also added an additional (possible) solution where you can have the plus button inside the list.
