Here's a screenshot to show the issue with testcase 3 (taken on my ThinkPad, connected to a high-res Samsung 27" display, with 200% pixel scaling configured in Ubuntu display settings). Note that the fringe goes away at some full-page zoom values. For me, it's present at 100%, 133%, 240% (as well as 80% and 67%).
Bug 1742583 Comment 11 Edit History
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Here's a screenshot to show the issue with testcase 3 (taken on my ThinkPad, connected to a high-res Samsung 27" display, with 200% pixel scaling configured in Ubuntu display settings). Note that the fringe goes away at some full-page zoom values. For me, it's present at full-page-zoom levels of 100% (default), 133%, 240% (as well as 80% and 67%).