As a user scrolls down the page using either a mouse wheel, trackpad or touch screen, they can become trapped by an inline scroll. Whilst trapped, they are still scrolling, but the page that they were scrolling before now becomes stationary. This situation can be likened to using a Treadmill, where you are moving, but your movement does not have any result in your position within the broader environment. In these situations, it is highly probable that the user's intention was not to scroll this subset of the page.
In order to exit the 'Treadmill', the user must either move their cursor or finger outside of the area the inline scroll occupies, or wait until they reach the bottom of that content, with scroll focus then returning to the main page. To "step off of" the Treadmill, so to speak.
How Inline Scrolls Come About
Content, such as text, occupies space on a page. If you constrain that content to a size smaller than it needs, then you are left with an overflow. Much like trying to pour a pint of water into a teacup. Eventually, it will overflow. In software, you are left with two options with this overflow when constraining; cut off the content, so the overflow is not seen (not really an option) or to allow scrollbars (potentially dangerous grounds!).
HTML elements that typically cause this are either 'iframes', or 'div' with a fixed height or width attribute value. The rational behind their use being to display content from an external site or system in the first case, or to minimise the presence of the content the div encapsulates, on the page.
What is The Problem?
Cursor or finger position on the page acts as a cue to the computer as to what the user would like to scroll. As the user travels down the page, what is below their pointer changes. This means that a user can inadvertently enter a region of the screen that has its own scrollbar, and then they would be scrolling that area instead of the containing screen. In the video below, there are three different examples of how cursor placement impacts the way the user scrolls the page:
What were the three errors when scrolling?
1. Cursor was not placed over a scrollable region
The location where the cursor is first placed in the video is not scrollable. Once the user eventually manages to scroll down the page, you will notice that the global navigation, actions bar, and heading remains in the viewport. In this case, they used a scrollable div element to make this, but instead they could have used CSS to achieve this "Sticky UI".
2. Cursor was trapped within an inline scroll
Once the cursor was placed in the middle of the screen, it began scrolling the scrollable region. Here the user got caught within the posts feed, but continued scrolling and eventually got to the end of that feed and scroll focus was given back to the parent container.
3. Cursor was trapped on the map
Finally, the cursor went down the left column, and ended up getting trapped within the map. In this situation, no matter how much the user scrolls up or down, they will never exit the map; they must first place their pointer outside of the map and then scroll, in order to continue scrolling the page.
This particular case study is a pungent one. When you begin to focus on where the user can scroll, you start to realise that there is not many locations on the page that they can safely place their cursor and travel down the page unhindered.
In the annotated screenshot above, the areas highlighted in green are areas the pointer can be placed without ending up on a 'Treadmill'. This means only 22% of the viewport is safe for a cursor to reside in. Should the cursor be placed in the remaining 78%, then the user will have to eventually move their cursor to one of the 'safe zones', which are not clearly marked like my annotated screenshot; they are not guaranteed to find the 'safe zone' in the first or second attempt either.
Moving a cursor to an exact location is not a quick action, as Fitts' Law will attest to. This all makes for a feeling of a loss of control, fiddly, and frustrating experience.
There are a number of design directions you can take to avoid or minimise the impact inline scrolling can have on an experience. Can we display on the page without the need of an additional scrollbar? Is the content list-like in nature, so that it can be paginated? Can we display it on request using progressive disclosure or a thumbnail?
In the Dynamics CRM 2013 example, the persistent header could have been solved by using a technique known as Sticky UI, which is a set of CSS rules. The posts feed could have either been paginated like the contacts section on the page, or had its own column and not introduce an additional scrollbar. Lastly, the map could have been an image, which when clicked, brings up a larger map view which is interactive; I would expect this would better fit the user's needs too, regardless of the Treadmill issue.
You cannot always avoid inline scroll. Many times, often to my displeasure, there has been a technical or business constraint which meant inline scrolling had to be used. But there are also common design patterns, like Master/Detail, which can make good use of this capability. In those cases, my advice would be to consider:
- Minimising the amount of screen width it or they occupies
- Use styling to differentiate that area, should it be a border, or background colour
- Do not nest inline scrolls within each other
Finally, as with everything you are doing, test it! In Axure you can make scrollable regions by placing content within a Dynamic Panel, and then:
Right click on the dynamic panel > Scrollbars > Show Vertical Scrollbar as Needed.
There is an option for horizontal scrolling too, but that should be avoided, especially if you are doing vertical scrolling as well.