Q3.5.10: Can I turn on sticky modifier keys?

Yes, with (setq modifier-keys-are-sticky t). This will give the effect of being able to press and release Shift and have the next character typed come out in upper case. This will affect all the other modifier keys like Control and Meta as well.

Ben Wing writes:

One thing about the sticky modifiers is that if you move the mouse out of the frame and back in, it cancels all currently “stuck” modifiers.