P: Brief flash on second monitor

Unfortunately this seems to be a complex problem, with many possible causes. I do not use HDR at all, and yet I have seen this problem for a long time. Much longer than just in Lightroom Classic 13.5 (I can't remember when it started, but I thin it was 13.0). I see it when I switch from Develop to any other module, or vice versa. I do agree it is GPU related.

I have a new observation on the main bug here, the disappearing/reappearing secondary display. It is specific to 13.5. With the secondary display in grid view and the main display in loupe view, use the left & right arrow keys to switch between images. Sometimes the secondary display would disappear as I advance from one image to the next, slowing me down and making a mess of things because of Secondary display takes keyboard focus, disabling some keyboard shortcuts.

I narrowed down the problem to navigating from SDR to HDR-edited images, and back. In a selection of only SDR images, no problem. In a selection of only HDR images, no problem. I have a mix of images which were converted to HDR in either 13.4 or 13.5, as well as lots of SDR. The HDRs edited in 13.4 have an SDR preview in loupe view, which was a separate problem I wanted to fix.

In the process of testing with a fresh catalog and writing up another bug, the problem went away in my main catalog. Yay for careful troubleshooting, I guess. No repro, no bug.

If someone else runs into trouble with HDR images in Loupe, here are some things I did which might or might not help:

• In "Preferences > Performance", disable and then enable "Use Graphics Processor: Auto".
• In "Preferences > Performance", disable and then enable "Enable HDR in Library".
• Set up a Library Filter for "Develop > HDR Edit Mode". Use the filter to select all photos with "HDR Edit Mode: On". Select all, then Delete Cached Previews for Selected Photos.
• Restart LrC.

@jasander77:

now in 13.5 the entire desktop space becomes inactive after each occurrence. Whereas the MacOS menu bar is grayed out and I must physically perform a mouse-click on my main display to reactivate the space.

I don't get the grayed-out menu bar, but this sounds like what I found with Secondary display takes keyboard focus, disabling some keyboard shortcuts. I agree on sticking with 13.4, though I don't see the connection to Mission Control. I never tried using Mission Control or Spaces with LrC. LrC has its own funky window management which, up to 13.4, works well enough on its own. There are reports of buggy interaction with Spaces elsewhere in the forum so I'm staying away from that.

In typical fashion ... I've had to uninstall the latest version (13.5) and go back to the previous version. This time due to further side effects of the program window cutting out. The program continues to be unresponsive during each occurrence, but now in 13.5 the entire desktop space becomes inactive after each occurrence. Whereas the MacOS menu bar is grayed out and I must physically perform a mouse-click on my main display to reactivate the space. Whatever is going on is clearly not playing nice with the Mission Control functionality and I am unable to tolerate this new annoyance and will remain in 13.4 until this is figured out.

I haven't checked this post in a while. Updated to 13.4 a few weeks ago and still experiencing the issue w/o any change.

Referencing the post by @clyde.c.brown. It's not random by any means. It is an almost a constant occurrence when using the Develop Module with the Grid display is on my secondary. Every time I flag/filter out an image in this situation the image disappears from Loupe on main screen and leaves just the gray background. A few milliseconds later the secondary LR window disappears showing my desktop background. LR is unresponsive until the window reappears. The next Loupe image will later reappear and the secondary will come back shortly after. The longevity of unresponsiveness can be random. Sometimes it's just a second or two ... other times it's several seconds. Interestingly, if I decide to undo that filtering with CMD+Z ... the image will reappear w/o causing flicker.

While I am in the Library module ... it does not happen at all. It also occurs everytime I switch between Library and Develop ... either direction ... regardless of what I have set on the secondary.

My workaround is to generally stay out of the Develop module when I need to do heavy flagging/filtering. For some reason,  I'm not adhering to that workflow this morning and LR keeps barking at me.

My issue disappeared with Lightroom Classic 13.4, but as far as I know this was never officially confirmed as a bug, and so it is unlikely that Adobe consciously fixed it. So I am beginning to wonder if this could be some third party utility interference, where that utility was updated on my computer around the same time that Lightroom Classic was updated to 13.4, and perhaps other people still use an older version of that utility. The big question is what utility this could be.

"Do you have an issue with one of those cables?"

I'm not sure what you're asking… I have an Other World Computing "Thunderbolt 3 Dual DisplayPort Adapter", so that's one Thunderbolt 3 cable from the laptop to the adapter, then two DisplayPort cables from the adapter to the external monitors. (Both are BenQ GL2706PQ.) The cables are new. I have been using the monitors happily for many years with my 2015 MacBook Pro.

With the MacBook Pro M3, I tested the OWC adapter with two monitors and with only one monitor. I also tested an HDMI cable direct from the laptop to one of the monitors, with no other peripherals connected besides a Bluetooth mouse. I tested the main Lr window on the external monitors and the laptop monitor. In all cases, regardless of which monitor has the main Lr window and which has the secondary, I get a disappearing secondary window. This didn't happen on the 2015 laptop. (Hmm, and I had the GPU disabled with Lr on that machine, since using the GPU made performance worse in Develop.)

My bug doesn't happen at random or when flagging images, like with @jasander77. It happens when switching between Library and Develop modules, in both directions, like @JohanElzenga found. And of course I switch between those modules often.

"dual Thunderbolt displays"

Do you have an issue with one of those cables?

The update to Lightroom Classic 13.4 (from 13.2) did not fix the problem with the disappearing second screen for me. Disabling GPU acceleration fixes it, but I just bought a top-of-the-line computer to run Lr faster—not to disable all those nice GPUs.

MacBook Pro M3 Max, macOS 14.5, dual Thunderbolt displays

Surprise, surprise! For me, the update to Lightroom Classic 13.4 seems to have fixed the problems with the secondary display. I can now use it again without any flickering when I switch between develop and grid...