As discussed in https://forum.kde.org/viewtopic.php?f=285&t=122478&sid=1c5ed6e881b392c388ec1a8f876737fc it would be quite appreciated from a usability point of view to introduce the pin functionality other plasma applets like calendar have in the widget explorer. Suppose the following: you want to place a widget and remove or replace it. As soon as you click the widget the widget explorer disappears. (this was not the case in plasma 1 btw). Now we have two options: 1.) always show widget explorer and never close it unless the user clicks the close button 2.) introduce the pin button The advantage of the pin button is that the user can decide, though it makes to close button redundant.
I agree with the problem description, but I don't agree with the proposed solution, since that would be treating symptoms instead of fixing the underlying problem: That Plasma doesn't properly separate configuration from usage. We seem to assume that people frequently change just tiny parts of their desktop, when in fact it is more likely that most users set it up and then leave it mostly as it is. Therefore we should introduce a clearly separated configuration mode, in which the widget explorer is always open. It should however by temporarily hidden when a Plasmoid is dragged over it so that it can be placed where it normally is.
I'm closing the bug as the Usability team is on it. I know they're working on a solution, so we'll treat this as a new feature.