MacBook Air display dims when video controls appear

Since disabling Dim Flashing Lights made no difference, there is no indication from that test that it is causing the problem.

The first thing to determine is whether the entire MacBook display becomes darker or only the video becomes darker when moving the pointer causes the video controls to appear. 

If the entire display dims, the issue should be investigated as a macOS display-related problem. If only the video changes, the video player or Safari is more likely to be relevant.

If the entire display dims, check whether Automatically adjust brightness is enabled in System Settings >> Displays and temporarily turn it off to test. 

Also test with Low Power Mode disabled, if it is enabled.

If only the video changes, play the same video in another browser, such as Firefox or Chrome. 

If the behaviour occurs only in Safari, temporarily disable Safari extensions and content blockers under Safari >> Settings >> Extensions, then test again. Apple explains how to manage Safari extensions here:

Apple — Change Extensions settings in Safari on Mac

You can also test the video in a Safari Private Window to determine whether the behaviour is associated with Safari's normal browsing data for that site:

Apple — Browse privately in Safari on Mac

I would not assume that macOS intentionally dims the display to make the video controls easier to see. I have not found Apple documentation identifying such a feature.

Because you are also missing several seconds of video and captions, note whether the brightness change and the loss of video/captions happen at exactly the same time. 

That detail is particularly useful because it can distinguish a display-brightness problem from a video-player behaviour.

Once those observations are established, the next troubleshooting step can be much more specific.

Hi,

To stop your MacBook Air display from automatically dimming or shifting brightness when you move the mouse and the video controls overlay appears, you need to disable a specific accessibility feature called Dim flashing lights.

When video control interfaces or play/pause buttons fade or pop onto the screen, macOS can misinterpret this sudden contrast or transition as a "flashing light" and aggressively dim the entire display to protect your eyes

Based on the symptoms described, this does not appear to be a change to the MacBook Air's overall display brightness. The rest of the screen remains at its normal brightness while only the video becomes darker when moving the pointer causes the playback controls to appear.

Similar video-darkening behaviour has been reported by Mac users, including with Safari and different macOS versions. Those reports do not establish the cause of the behaviour on your Mac.

Your testing shows that disabling Automatic Brightness and Low Power Mode does not change the behaviour, and that it also occurs in Chrome. Since you have also experienced it across multiple macOS versions, the available evidence does not point specifically to Automatic Brightness, Low Power Mode, one browser, or macOS 27.0.

The dimming occurs when the playback controls appear, but that timing alone does not establish what causes it. It could be related to the particular video player, browser behaviour, or how the video and its controls are presented.

Because the captions remain visible but become too dim to read, another useful test is to turn captions off and then back on while reproducing the problem. If the captions and the underlying video change brightness together, that would indicate that the dimming affects the video presentation as a whole. If the captions behave differently, that would provide additional information about how the player is rendering them.

A further Safari test is to use Video Viewer, if available, and then Picture in Picture. Apple documents both features for supported videos. (support.apple.com) If the video remains at normal brightness in one of these modes while it dims in the normal webpage player, that would provide useful evidence about where the behaviour occurs.

Testing several unrelated video sites would also help distinguish between a player-specific behaviour and something that occurs more broadly. If the dimming occurs only with certain players, that would point toward those players. If it occurs consistently across different players in both Safari and Chrome, the cause is less likely to be specific to one website.

At this point, there is not enough evidence to identify the behaviour definitively as either an Apple bug or an intentional feature. Based on the testing already performed, further changes to Automatic Brightness or Low Power Mode are unlikely to address this particular symptom.