Created attachment 689616 [details] Screenshot I noticed this with the Social API sidebar, but it turns out I just never noticed it on history/bookmarks. I'm using the firefox_blue lightweight theme, and the splitter have a really weird coloring: a light zone at the top and a black zone on about five sixths of the height. See attached screenshot.
Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336
Component: XP Toolkit/Widgets: XUL → XUL
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