Looks like I get similar looking log whenever first scrolling using touchpad/two fingers and then stop scrolling. But when the painting stopped for longer period, the log was stuck at HiddenWindowCallback.
Bug 1951249 Comment 16 Edit History
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Looks like I get similar looking log whenever first scrolling using touchpad/two fingers and then stop scrolling. But when the painting stopped for longer period, the log was stuck at HiddenWindowCallback. (I'm trying to capture another log to ensure it looks similar to the one in the previous comment.)