The prevalence of the acronym tl;dr ("too long; didn't read") suggests that people intentionally disengage their attention from long sections of text. We studied this real-world phenomenon in an educational context by measuring rates of intentional and unintentional mind-wandering while undergraduate student participants (n = 80) read academic passages that were presented in either short sections of text (one sentence per screen) or relatively long sections (2-6 sentences per screen). We found that participants were significantly more likely to "unintentionally" disengage their attention while reading the longer sections of text, whereas intentional mind-wandering rates were equivalent across short and long sections of text. The difference in unintentional mind-wandering rates suggests that section length may serve as a cue that people use to assess the cost-benefit tradeoffs involved in attending to (or disengaging from) text. We conclude that instructors should avoid presenting electronic reading material in long sections of text.