I don’t reboot often, so I haven’t found a pattern. I did reboot today, and the MacBook Air only used the internal display. It was in clamshell mode, so I was looking at blank screens. (I have two external displays.)
I opened the clamshell, and one external display became active. (My understanding is that the MBA M3 can drive one external plus the internal, or two external displays while in clamshell mode, so I wasn’t surprised.) I closed the clamshell, and both external displays became active, (All my windows had migrated to the primary external display; that was a bit annoying.)
Sometimes, rebooting just works. In other words, both external displays light up and are available. Sometimes, I need to open the clamshell, like today. And sometimes, I need to open the clamshell and allow the MBA to use the Thunderbolt hub that is between the MBA and both displays.
What determines the different behavior? As far as I know, I just select Restart from the Apple menu, and the computer has different behaviors.
Not much help here most likely. In my simple experience, when I power up a computer with external monitors, the computer sends a signal to the monitor saying “are you there?”. If the computer doesn’t hear back what it wants, well then it assumes the external monitor isn’t there. With third party monitors and the type of connection you have (DVI, USB-C, HDMI, etc.) I find it is pretty easy for the “are you there and who are you?” question to get lost. Especially if the monitor went to sleep and was slow to reply. Making sure the monitor is awake and ready to respond often helps.
What you say makes sense. In this situation, however, I was choosing Restart from the Apple menu. Both monitors were awake when I chose Restart, and I doubt (but don’t really know) that they went to sleep in the time it took for the Mac to restart. Many weeks ago, to try to reduce the chance of accessories muddying waters, I plugged the Studio Display directly into the Mac. That did not result in consistent behavior. In other words, sometimes the Restart happened as I desire (both monitors in use) and sometimes not. Since I want to believe that computers are deterministic, it’s frustrating.
Grasping at straws here. On my M1 Max Studio, a restart takes a few seconds. That is, I use the Apple > Restart command. The Studio asks me if I really meant to do that. I tell it yes. It seems to do nothing for a moment. Then the screens go blank. I see one of my two monitors display a “I am going to sleep now as there is no input” message of some sort. Then after a beat or two, I hear the PONG and one monitor comes up right away and the one that gave me a message takes a few seconds longer to come back on.
What I am grasping at is that each monitor has its own sequence it must go through after it detects “signal gone.” While software and firmware are deterministic, the timing isn’t always the same. If one of the monitors is still executing the end of its “time to sleep since the signal went away” when the computer sends the “I’m baaaaccck” message, it can easily be that the ships cross in the night and nobody is talking to each other anymore. I find that HDMI connections have more of this issue than DVI connections. I have no experience with Video over C connections.
It could also be that your computer is haunted and all bets are off.
More seriously, thanks for you thoughts. The Studio Display does not give an indication when it goes to sleep, so I don’t know how fast it is. The other display (Acer) does give an indication, and it does not go to sleep during a restart. It is the secondary display when both displays are awake, but I don’t think that would prevent it from showing something.
There are enough other oddities that I think you nailed it with haunted—or possessed.