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Usually I keep my widgets locked since I find it distracting that the side-panel of them shows up when the mouse pointer is hovering over them if the widgets are unlocked.

However, sometimes it would be handy to manipulate the size or location of a plasmoid. It's of course possible to just unlock the widgets, but it would be more convenient to just hold down a qualifying key and the plasmoid that's under the pointer is temporarily unlocked while keeping the rest locked.

Excellent idea. I have to second this. I use a number of widgets which display different amounts of data, depending on my input (e.g. the dictionary plasmoid). Since I have few screen space, quite often, I want to temporarily resize one of these applets. Doing this with a meta key combination and mouse dragging would make the interaction much more natural.