While dragging a widget, it is reparented to "root" so it can be moved freely without the other applets reacting on that. However, in right-to-left mode, the toolbox is on the left side, the coordinate system of the applet parented to "root" did not match with the applets in "currentLayout".

This happened to work in left-to-right as there "currentLayout" was at coordinates 0,0. Instead of manually fiddling the margins into the positioning, just reparent the widget to another Item which is anchors.fill'd to the layout (and thus has its margins) and let QML figure it out.

Moreover, mirror the flyout that shows the applet name and config/delete buttons.

I can now properly re-arrange widgets in a horizontal panel with

plasmashell -reverse

When the applet is over the right-most widget (Kickoff in the default case) it glitches slightly but that's still way better than placing the applet at seemingly random positions when letting go of the mouse pointer.