If I open the widget explorer and I want to close it again, I have to press the small close button in the right corner of the panel to close it. I would expect that clicking to the desktop is enough to close the widget explorer, probably it is designed in this case as a feature, but it is not the same behavior like the settings of the plasma panel. From my point of view the widget explorer should close like the plasma settings panel by clicking to the desktop and it would work better on a touch screen as well. Tested on Plasma 5.12.3 on Wayland.
Then you can't add multiple widgets and move them about.
Thank you for your feedback. I just wanted to tell my personal preference. On Android it works the same, you add a widget and the widget explorer closes automatically. And thus clicking to the desktop to close the widget explorer would be fine as well. It seems convenient especially on tough devices. From my point of view adding multiple widgets is a rare use case, but I accept that this wont be changed, thanks again.
Fixed in Plasma 5.19 beta.