I did some further investigation into my settings in the adblocker / exceptions / lists area because I think that's where the problem is:
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I found something that makes a small difference if I remove it. It's a list I added in 2024 but I haven't had any problems with it so far:
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If I remove this list, the videos load when the page loads (the video frame is no longer black without the video) and can be started, but the other problems (which I think are related to Opera's behavior) remain:
when the page loads, the videos start by themselves, without me giving the "play" command
when the page loads, the videos are on "mute" and must be put on "unmute"
when scrolling down the page, when the video clip goes off screen, a pop-up appears with the clip in the lower right corner of the page, without me asking for this
when scrolling down the page, when the video clip goes off screen, a pop-up appears with the clip in the lower right corner of the page, which restarts even if the clip in its main frame was stopped, without me asking for this
I mention that all the settings for detached video and for Video Popout | Picture-in-picture video are disabled, but Opera does not apply these settings:
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These are unwanted behaviors that should not exist.
I would like the videos to be stopped on page load (and not start on their own even if I scroll the page) and to be unmuted. Now it is the opposite.