I switch from Unity ti KDE for months, and I still not get the shortcut of Plasma Widgets.

A lot of widgets just do nothing when I press the combination,
some certainly react, for example, the Network Manager,
but what bothered me is that you actually cannot do any further operation with the keyboard,
then I think it just lost any point of the shortcut.

Moreover, Same situation happened when I used the show desktop feature,
after the desktop(Folder View) showed, I can do nothing with the folder view widget, sometimes the file icon can get focused, but sometimes not,
the "menu" key can only pull the context menu of folder view widget instead of the file's,

In general, there seems nothing can be done with only keyboard, which is well implemented in Unity and former gnome.

Was it designed into this way(which seemed to be a little bit dumb), or I just miss some configuration?

It is likely an oversight in the implementation of various Plasma applets. I would suggest filing bug reports, or using the tab key to attempt to navigate around the desktop - as it may offer some keyboard navigation functionality.

This problem still bothers me,

Anyone get a [compromising] solution?

or, just vote the bug.
https://bugs.kde.org/show_bug.cgi?id=324057

It seemed that problem is little bit trivial, so won't be fixed for a long time.

Anyway I find the xdotool is pretty handy, it can simulate the mouse actions, anyone got the same trouble could give it a try.