Long-clicking is in general not something that you do with a mouse, and it only makes sense with touch-based input. Therefore resizing plasmoids with a mouse, which requires opening the handle containing the resize button, is very hard to figure out.
With a mouse pointer, the behavior from plasma4, where the handle is shown when hovering over the plasmoid, makes a lot more sense.
Minor input as I directed the user here, due to this coming up frequently enough in #kde on IRC:
One possibility would be to at least add an entry to the context menu which opens that side bar or have a submenu with the possibilities of the side bar (move, resize, rotate, close).
Opening the context menu is rather intuitive and well known on desktop systems.
Very much agree.
While this is reasonable for touch devices, it is very non-intuitive when using mouse input. The KDE4 way worked very well for me.
In fact one can enable the KDE4 behavior by deselecting the checkbox "Press and hold widgets ..." in the plasma workspace settings (tab "Tweaks").
This option works fine (Plasma 5.7.4), only IMO it takes a bit too long for the handle to appear.
As long as this function stays, this bug probably can be marked as resolved.
Fixed with the awesome new global edit mode in Plasma 5.18!