Apply regionprops() to find the bounding boxes. Sort the bounding boxes by lower left corner, according to x within y (that is, y first, and if there are multiple ones with the same y, then sort those ones by x.) This is equivalent to sliding your attention column by column (x coordinate) within each row looking for a lower left corner of a bounding box. (Depending on the scan order, you might prefer to add the height to the y coordinate so you sort by upper left corner.)
No actual sliding needs to be done for this once you have your bounding box coordinates from regionprops.