Oh right, yes, I see what you mean. I have Acon Digital DeNoise and it has the resize handle as you mention. That's good to know.
However there's definitely something that Resolve doesn't support that Reaper does, eg in the case of FabFilter's VST3 plugins. FabFilter don't provide a drag handle, they instead adapt to the host window resizing. But as Resolve's plugin window can't be resized, the plugin also can't be custom resized. It can however be changed to one of the Small/Medium/Large/Very Large preset sizes.
It would be nice if Resolve supported dragging the plugin window, so that plugins like FabFilter that detect that would adapt to it. Perhaps a "Allow resizing plugin window" tickbox in the three-dot menu?
It's not a huge deal, just a nice to have.