To make effective use of the screen, I deleted all panels and removed the cashew using the most recent version of the "ïhatethecashew".
The few useful widgets (pager, clock and keyboard layout switcher) are freely positioned in the desktop's lower right corner because they are least intrusive there.

I want these widgets to be always on top, always visible and never getting obscured by other windows.
On the window menu of "normal" windows one can define that a window always stays at top (i.e. other windows go below it).
However, the widget windows lack such a menu and they don't have an according option in their settings menus either.

So, any idea how to make widgets stay above other windows?

Widgets on the desktop are part of the desktop window (which is by definition the lowest item in the stack) - they cannot be put above other windows, as they are not.

You'll have to use the "plasma-windowed" containment, eg. "plasma-windowed --noborder clock", what will add a window and (in this case) put the analog clock applet into it.
You'll need such process and window for every plasmoid you want to add.

Those are "normal" windows and you can use kwin rules to put them onto a certain position, size, virtual desktop, keepAbove etc. (you'll have to match the title to distinguish them)

Great, this made the pager widget usable for me! Big thanks!

BTW, meanwhile I found a similar solution in this thread viewtopic.php?f=67&t=83850 using "plasmoidviewer pager".
But after I tried to make title bar and borders go away using the <window menu> - <More Actions> - <Special ...> sequence, I could not move the pager window again, and I didn't succeed in getting the title bar and borders back, ALT-F3 somehow didn't address the correct window (i.e. plasmoidviewer).
When exiting and restarting "plasmoidviewer pager" they were still gone, rendering this workaround unusable.

I am now somewhat afraid to remove the title bar and borders of plasma-windowed in fear of getting the same problem again, as I am really happy to have a functional pager now (with a title bar though, but this is a minor problem for now.

A long term solution could be to merge the old kpager code (I like the pixmap mode ) with the newer, fancy pager widget code. Already downloaded both sources and looking through them.

Thank you again, Luebking!

You can move/resize windows by pressing Alt and using the left/right mousebutton on them.

Alt+F3 operates on the active window. From what i can say, at least the clock plasmoid spawns an activatable window.

The "--noborder" switch alone controls whether the window has a border or not (and is translucent, btw), so you don't need to create a window rule for that at all.

All special window/applications settings are accessible through "kcmshell4 kwinrules", you can remove the rule on the plasmoidviewer that removes its border there.

Wow this was plenty of very helpful information again!
Good to know how to reverse mistakes like that one with the window menu.
Thanks again Luebking!