I can't provide the reason, but I can add a story that might shed light on the answer.
Years ago I was hosting a movie night and we were watching a dark, stylish black & white film that required subtitles. This was either in iTunes or TV.app, whichever was Apple's flagship movie watching app at the time. Every time a subtitle appeared on the screen, the gamma (or something affecting brightness) would change ever so slightly, affecting the whole scene in a barely perceptible way. It drove me nuts. This wasn't some kind of adaptive dimming from the screen; I confirmed that the actual image (even when watching just on the laptop screen) was changing.
After some searching, a really wacky solution popped up: if you go into the Accessibility settings and bump up the size of the cursor ever so slightly, it'll fix it.
Supposedly, the reason was that macOS's cursor drawing is heavily optimized (potentially in the firmware?) and thus uses a different drawing path (maybe with GPU acceleration vs. without?) depending on whether the default cursor is being used or not. So, whatever drawing routine was used to render subtitles causing the gamma to shift was also triggered by having a different default cursor, making it the same whether or not subtitles were on the screen.
Even though I found a fix, to this today I'm still furious that Apple – supposedly known for their quality, attention to detail, and good multimedia experience – could overlook this in their flagship app. And now it's something I always do when setting up a new Mac: bump the cursor size.
That's a bug in Intel GPUs Drivers. Windows Server can use an optimized display mode when there is only a Core Animation Layer that covers the entire screen. Unfortunately on Intel GPUs color management is not correct if the layer bit depth 8-bit.
Every time a subtitles line appears, the Windows Server switches out of the optimized mode, and so the color management corrects itself, and when the subtitles line disappears, it goes back to the optimized and buggy mode.
The good news is that Intel GPUs are not used anymore on recent Macs, the bad news is that Intel GPUs drivers will never be fixed.
Probably the enlarged cursor prevents the usage of the optimized Windows Server mode.
Just my guess. But it's been introduced at least 6 years ago, and Apple switched to its own CPU and GPU three years ago, so each passing year there are fewer chances of it getting fixed, and relatively soon macOS will require and Apple GPU.
Is that not just "adaptive gamut" or whatever it's called — where a display capable of HDR (like the recent miniLED-driven iPad displays) will still be driven with an SDR signal most of the time to save power (as SDR content has a lower contrast ratio, and so is less demanding of peak brightness), but will switch into HDR mode when displaying HDR content (and so "desaturate" — i.e. bring down the black level, lowering the seeming contrast level for darker content — in order to display bright lights as really really white)?
It's maybe strange that compositing HDR + SDR together results in SDR rather than HDR, of course. You might consider that to be the bug.
But I could see that being an intentional choice by Apple (though one that's maybe not thought out in the context of an iPad-as-consumption-device.)
In most situations where you're staring at a composited HDR+SDR signal for a long time — e.g. a video editor editing an HDR video timeline/preview, inside an otherwise-SDR UI — the SDR content exists both before the HDR content is opened, continues to exist after it closes, and takes up the majority of the screen. So you don't want the SDR content changing its gamut just because some HDR content shows up. You want the HDR content to be squashed down to SDR for preview. If you want to check how the video looks in HDR, you'll preview the video fullscreen for a bit. (Or, if you want to always be seeing the video in HDR, then you'll use a dedicated second display that the video is always fullscreen on — as any color-grading software expects you to do.)
The scrubber widget being composited on top of HDR content is an exception to this, of course. That widget should be brought into HDR, not the other way around. But maybe this is a very low-level compositor thing, that's hard for something all the way up at the app level to signal down to to say "hey, don't mind me, we're watching a fullscreen video up here, just make me HDR!"
This happened to me too. I still use a MacBook Pro from 2015 mostly for watching videos. As you described, any time a subtitle appeared on the screen, the screen fluctuates in brightness / gamma levels.
I guess one other thing you could say is, I don't know if it's just me in my head, but I always feel that the cursor movement in MacOS is silky smooth.
Based on my experience with the Mac tv app, I’m guessing no powerful people at Apple dog food it… tons of little wierdnesses compared with the AppleTV proper.
(It’s been a long time since iTunes played movies, but it was pretty bloated and weird before they popped TV outta there)
Years ago I was hosting a movie night and we were watching a dark, stylish black & white film that required subtitles. This was either in iTunes or TV.app, whichever was Apple's flagship movie watching app at the time. Every time a subtitle appeared on the screen, the gamma (or something affecting brightness) would change ever so slightly, affecting the whole scene in a barely perceptible way. It drove me nuts. This wasn't some kind of adaptive dimming from the screen; I confirmed that the actual image (even when watching just on the laptop screen) was changing.
After some searching, a really wacky solution popped up: if you go into the Accessibility settings and bump up the size of the cursor ever so slightly, it'll fix it.
Supposedly, the reason was that macOS's cursor drawing is heavily optimized (potentially in the firmware?) and thus uses a different drawing path (maybe with GPU acceleration vs. without?) depending on whether the default cursor is being used or not. So, whatever drawing routine was used to render subtitles causing the gamma to shift was also triggered by having a different default cursor, making it the same whether or not subtitles were on the screen.
Even though I found a fix, to this today I'm still furious that Apple – supposedly known for their quality, attention to detail, and good multimedia experience – could overlook this in their flagship app. And now it's something I always do when setting up a new Mac: bump the cursor size.