I believe the state sequence is as follows, under Windows 10:
- First press - engage the modifier key for the next (non modifier) keypress
- Second press - lock the modifier key
- Third press - cancel the modifier key
Multiple modifier keys can be engaged with single or double presses.
Edit Feb 2021
I have recently tried to reproduce these findings and it appears to be keyboard dependent. I tried two keyboards. One works exactly as described above. The second
works like this:
- First press of either shift - engage shift modifier for next key (alpha or numbers)
- Right then left shift in sequence - lock shift function. Single left shift to clear
- Control, alt - just modify the next keypress.
The sticky keys icon on the taskbar does show what modifier will be applied. This is quite fiddly, and I can't say if it is unreliable or I get in a muddle, but the next press is not always what I expect. If it is reliable, I think I could get used to it.
I cannot find any reliable sticky keys information from Microsoft. If you can reference good information, please comment.
For what is is worth, sticky keys works much better for macOs and an iPad with an external keyboard.