A drop-down list is a user interface element which allows the user to select from a list of mutually exclusive options. It appears as a button which, when clicked, reveals a list.
Radio buttons or a list will often be preferable to a drop-down list, as they present all the available options at once without any further interaction. At the same time, a drop-down list might be a better choice when:
The number of options is large.
There is little available space.
The list of options may change over time.
The contents of the hidden part of the menu are obvious from its label and the one selected item. For example, if you have an option menu labelled “Month:” with the item “January” selected, the user might reasonably infer that the menu contains the 12 months of the year without having to look.
While GTK+ does provide a specific combobox widget, it is recommended that drop-down lists are constructed using a combination of a button and a popover. This allows more comfortable scrolling for long lists, and allows search to be integrated into the list. Using this approach also allows the current selected item to always be displayed.
If the number of items is very large, provide a search function that filters the list.
Label the drop-down list button with a text label above it or to its left, using sentence capitalization. Provide an access key in the label that allows the user to give focus directly to the drop-down list.
Use sentence capitalization for drop-down list items, for example
A drop-down list can allow custom values to be added to a number of presets, or can be used to enter and select from custom values alone.
Ensure that custom values are ordered in a way that is most useful to users. Alphabetical or recency ordering is common.
When a drop-down list includes both preset and custom values, separate them into separate groups within the list.
Allow custom values to be removed from the list.
Validate custom values as they are entered, in order to prevent errors.
If the drop-down only accepts custom values, and no values have been entered previously, present the custom value entry when the drop-down is opened, instead of showing an empty list.