On my Shift phone you can open the power button menu by long pressing that button - so far so good (and normal).
Unfortunately the huge size of the phone has side effects. When also holding something else in the same hand is happens to me not that rarely the I press the power button by accident and eventually a restart is initiated.
Is there an option to enable a confirmation dialog for such actions?
I am having a bit of trouble picturing your sitaution: so you have something else in the same hand as your phone (maybe a small notebook or something) but your thumb is resting on your power button so it accidentally gets “long pressed”? Maybe you are running and holding your wallet too?
Would a notification appear on the screen (are you seeing the phone screen)? Again I think I am missing something. Maybe you don’t see the screen, but in that case what good could a notification be? Maybe you want an audible notification?
Usually I carry more than one phone (like private, work and one for development).
When holding them I hold them stacked up, hold them on the edges and only softly touch the screen. Because the Shiftphone is the biggest one in order to firmly hold on to the other phones I need to press a press. The the very soft pressing point of the power button then opens the power-button-menu.
Add to that the known problem where one touch of the screen turns it on (instead of a double-tap).
I wasn’t suggesting a notification, but more like a confirmation dialog like “Do you really want to power-off/reboot?”.
OK understood. But if the confirmation dialog comes when the screen is hidden by another phone it will not help, right? By hardware design I believe a long long press will always trigger a reboot since if the system crashes it it always possible to reboot.
But surely there must be a more satisfying solution than “you are holding it wrong” 
Anyone have more ideas?
About the screen waking, you can try to de-select “Tap to check phone” and / or “Lift to check phone” both under “System Settings > Display > Lock Screen” which may help a bit?
If that’s true, then a confirmation for this would indeed be impossible. I fully support the idea of a repairable phone, but the disadvantages of the size and weight are really uncomfortable.
Yes, that might help here. The thing is that despite its frequent unwanted activation I still prefer this slightly over having to use the power button every time. I hope they’ll fix this eventually.