Fortrikka You have mentioned in the post specifically for horizontal tabs
On horizontal tabs, middle click only closes tabs if you press it over the highlighted field, and finally close it only when you release the middle mouse button over the highlighted field.
And seams like you are testing on vertical tabs now. Both are different. Or please edit post to indicate both vertical and horizontal tabs, as its quite confusing for all of us.
I request you to please test horizontal tabs and confirm if that's working as expected (relevant to this post). So that we can apply same fix for vertical tabs too