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Reader Bar marks the previous scroll position (the screen area visible prior to scrolling, to be exact). It helps to keep track of your reading position, especially when scrolling at the very bottom or very top of a page, when scroll steps are not integral.

Available in userscript and bookmarklet flavours. Tested with Firefox and Firefox w/ Greasemonkey.

Changes from 0.2 to 0.21: Added z-index to the marker element, to make it work right on sites like Wikipedia.

Update Jan 10th: The marker currently blocks clicks in a part of the page. This will be fixed shortly.