[Prerequisites]: - Have the "Bookmarks" sidebar open. [Steps to reproduce]: 1. Open the browser with the profile from prerequisites. 2. Resize the sidebar and observe the position of the arrow next to the "Bookmarks" text. [Expected result]: - The arrow position remains unchanged. [Actual result]: - The arrow is slightly moved. [Notes]: **On Mac 10.12.**: - Using latest Nightly build 58.0a1 (10/08/2017) the same behavior can be observed for "Synced Tabs", and not for "History". - Using latest Firefox release 56.0, the same behavior can be observed for "History"; and not for "Synced Tabs". **On Windows 10 x64**: - The issue is only reproducible for "Bookmarks" using latest Nightly build 58.0a1 (10/08/2017) and Release 56.0; and not for "History" and "Synced Tabs". - The issue is not reproducible on Ubuntu 14.04 x 64 using latest Nightly build 58.0a1 (10/08/2017), nor Release 56.0. - Attached is a screen recording of the issue.