Joining areas can be very finicky at times because of the need for everything to line up perfectly.
In the simplest cases, this is not a problem. But as your workspace gets more complex with more areas, you might need to manually line up one edge with another in order to start a join. But alignment must be PERFECT. It can look seem perfectly-aligned but will not join if out by a single pixel. Especially difficult if using a tablet pen or if using a high-DPI display.
This patch makes the join a bit more forgiving. Basically adds a bit of wriggle room so it can proceed as long as the alignment is close.
The "wriggle room" is quite small in order to avoid some potential problems with very small areas. In a nutshell you still have to get the edges close. The following image shows two areas aligned well-enough vertically to do a horizontal join (assuming the other horizontal edges are similarly close to each other)