Removing this line fixes the line height changes: https://dxr.mozilla.org/mozilla-central/source/devtools/client/shared/sourceeditor/codemirror/lib/codemirror.css#273
Bug 1576163 Comment 3 Edit History
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Removing this line fixes the line height changes: https://dxr.mozilla.org/mozilla-central/source/devtools/client/shared/sourceeditor/codemirror/lib/codemirror.css#273 Correction: Removing it also causes the line-widget to overlay the code. Bad hack, but just removing also is bad.