OOjs UI v0.19.4 changed some things that affect the scale of buttons and therefore also the arrows in the RevisionSlider. See the screenshot from the current master:
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Change 340718 had a related patch set uploaded (by WMDE-Fisch):
[mediawiki/extensions/RevisionSlider] Fix arrow css for OOjs UI v0.19.4
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Change 340793 had a related patch set uploaded (by Addshore; owner: WMDE-Fisch):
[mediawiki/extensions/RevisionSlider] Fix arrow css for OOjs UI v0.19.4
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Change 340718 merged by jenkins-bot:
[mediawiki/extensions/RevisionSlider] Fix arrow css for OOjs UI v0.19.4