Determines the number of lines of a mini-index which is shown when in the pager. The current message, unless near the top or bottom of the folder, will be roughly one third of the way down this mini-index, giving the reader the context of a few messages before and after the message. This is useful, for example, to determine how many messages remain to be read in the current thread. One of the lines is reserved for the status bar from the index, so a pager_index_lines of 6 will only show 5 lines of the actual index. A value of 0 results in no index being shown. If the number of messages in the current folder is less than pager_index_lines, then the index will only use as many lines as it needs.