The quoted tool-tip obviously is misleading, as there is no drawing over the main editor view, but rather on the zoomed preview in the sidebar.
Please have a look at following GIF. Note the main editor view (MEV) and the white overlay surrounded by gray overlay on the overview-sidebar to the right. Line-wrap is enabled.
- Frame 1: Overlay in Overview sidebar is on top, showing ASCII-art "1", "2" and "3". MEV only shows "1". - Frame 2: Overlay in sidebar is at bottom and only two thirds the usual height (!), overlaying "9" and "10", while MEV only shows "4". - Frame 3: After scrolling down a little bit, overlay in sidebar has sidebar's full height, no gray overlay anymore (this also unexpected). In that sense, MEV only shows "5", while overlay shows "1" to "10". - Frame 4: Scrolled to the very bottom in MEV, which shows "10". No change for Overview's overlays, same as in frame 3.
For all frames the overlay for the Overview plugin's sidebar does not reflect the same quantity of lines as the main editor view does. However, in my understanding reflecting the main editors view is claimed in the preferences dialog.
For frame 3 and 4 the overlay in the sidebar does not provide any information. Scrolling between the states presented by both frames does cause any change in the sidebar, which also was claimed differently by the plugin.
The plugin claims to also be a visual reference, as denoted in the tool-tip description. With line-wrap enabled the overlay implementing this visual-reference-feature gets heavily out of sync.
So even if you insist on the out-of-sync-ness working as intended, you can at least see that for frame 3 and 4 the overlay is behaving strangely.