Right-clicking on an edge between editors brings up a context menu for "Area Options".
One item is shown as "Split Area", but I find that a bit unclear. You have clicked on a space between two areas so referring to a single area is confusing. Similarly an area can be split either horizontally or vertically and this will only do one of them, depending on the orientation of the edge. But that is not communicated to the user. This patch changes this item to say "Vertical Split" or "Horizontal Split" depending on what it would create. This should help lead the user into guessing how to do the other split direction.
The other item is "Join Area", which has the same problem. Join which area to where? This patch shows it as "Join Areas" and only shows this item if such a join is possible.
Following shows the menu at a horizontal edge where joining is possible:
And the following shows the menu at a vertical edge where a join is not possible:
And this also changes the border width thresholds so joins actually work at larger resolutions.