There should be some permissible threshold when joining shapes.

[sagarjauhari]

When joining shapes, there could be a permissible threshold for the distance so that the shapes would be joined even if the edges were not exactly overlapping but close enough (within that threshold)

[yqlu]

There is indeed a fuzzy equals for detecting whether two edges should be joined -- see here, which calls approxEqEdges (which has a default tolerance of 1 pixel).

In my experience this tolerance is sufficient to automatically join two edges which visually look like they should be joined. Do you have an example where it is too low?