There are many ways that a user interface can react to mouse events. The most commonly used mouse events are:
- Mouse click. Used for following a link, activating an icon, or selecting an item.
- Double click. Used mostly for opening a program or highlighting a word.
- Triple click. Highlights a paragraph of text.
- Right click. Used for opening a context menu.
- Middle button click. Rarely used because not all mice have three buttons.
- Scroll wheel. Used for scrolling text. Not all mice have scroll wheels, but they are much used if present.
- Mouse-over (hover). Used mostly for changing the mouse cursor on objects that can be manipulated and for showing pop-up descriptions about objects.
- Mouse drag. Used mostly for moving objects or boundaries.
- Highlight by moving mouse with button down. Used for selecting text.
- Shift key + click. Extends selection.
- Ctrl key + click. Selects multiple items.
Most of these effects are not visible, and only proficient users are aware of all of them. However, it is good to support these effects because users may expect them to work.
Many user interfaces, such as office programs and websites, are so crowded with mouse effects that it is difficult to avoid activating them inadvertently. A user may click in a window just to move the focus to that window or place the cursor. But sometimes it can be difficult to find a place to click that doesn't do anything else than that.
Double clicking is inconvenient for many users and may be replaced by a middle button click (see Mouse).
Mouse-over effects can be very disturbing, especially if the user interface is so crowded that it is difficult to find a place to put the mouse that doesn't do anything. A good alternative is to make a right-click context menu with a "What's this" option.
A mouse-over effect is useful when it changes the mouse cursor to indicate a link that can be clicked or a border than can be dragged. This effect should be immediate. But the mouse-over effect that pops up a message should be delayed for approximately one second in order to avoid too much flickering.
The mouse-over effect may change the color of a link or underline it, but never change the font size. Any mouse-over effect that changes the size of an object can cause everything below the resized object to move, which makes the whole image flicker when the mouse is moved around. This is very disturbing.
Some programs have additional mouse effects that are disturbing. Clicking on a filename in Windows will select the file. But if the file is already selected, the click will rename the file. This is unexpected and may lead to inadvertent renaming of files. Some word processors will pop up a context menu when a word is highlighted with the mouse. This is disturbing because it covers some of the text. Often a user will highlight a word just to use it as a fixation point to the eye. Any side effect of highlighting the word is unwanted in this case.