Apple Forgot About Fitts' Law

I have this quirk when I’m reading something on my computer (website, PDF document, etc) that I want to keep my mouse pointer visible so I easily know where it is when I want it, but I don’t want it in the content area itself obscuring what I’m trying to read. The margin of the content is the perfect place for this and that is generally where I leave my mouse when reading.

The other day I opened 5 PDF documents in the same Preview window to read, putting that window into Lion’s handy full-screen mode.

When I went to place my pointer in the left margin of this preview window, however, it wound up hitting the left edge of the screen causing a nifty navigation pane to slide out. I thought to myself “Cool!” and took a mental note for later when I had to switch between the PDF’s.

I was then putting my pointer back in the left margin, being cautious not to hit the edge, when the pane appeared again. This had me very confused and after a few seconds of experimenting I found out that a 40 pixel column along the left edge of the screen (give or take a few pixels) is the trigger area for the pane — not the edge of the screen itself.

This design goes directly against Fitts’ Law. If you don’t know what Fitts’ Law is, I recommend taking a look at the Wikipedia entry (Success and Implications section) for a quick summary. I want to draw attention to one point in particular from there:

Edges and corners of the computer monitor (e.g., the location of the Start button in Microsoft Windows and the menus and Dock of Mac OS X) are particularly easy to acquire with a mouse, touchpad or trackball because the pointer remains at the screen edge regardless of how much further the mouse is moved, thus can be considered as having infinite width.

This means making the trigger extend inside the edge of the window is unnecessary due to the infinite depth this pane comes out of. This is due to the fact that once I know the left side of the screen activates this pane, I am not going to carefully place my pointer there, I’m going to “throw” it left, letting it hit the edge (as I do now). This is a similar action to a person showing a hidden Task Bar (Windows) or Dock (Mac OS).

The only reason I can think of for Apple extending the trigger area into the content is to increase discoverability — a person might be more likely to move their pointer near the edge of the screen than actually hit it. Also, unlike the Windows Task Bar and Mac OS Dock which are visible by default, there is no indication that this pane even exists until you activate it. However, this design comes with a major consequence beyond the annoyance for my quirky mouse-placement ways: if there is ever content under that trigger area, it is inaccessible for annotation or selection (despite the margin being prime real estate for annotations)¹.

I have high respect for Apple’s design and execution in their software and hardware, but I believe something was overlooked here, or I am not seeing a use-case that this design is the solution for.