[Affected versions]: - 51.0a1 (2016-09-18) [Affected platforms]: - Windows 10x64, Mac OS X 10.11.1, Ubuntu 16.04x64 [Steps to reproduce]: 1. Launch Firefox. 2. From about:config, enable the devtools.responsive.html.enabled pref. 3. Enable RDM. 4. Choose '4K Ultra HD Television' device. 5. Scroll the page to the right, only until the device button is displayed. 6. Choose a small device from the list (Eg. Samsung Galaxy S5). 7. Open the devices list. [Expected result]: - The list is correctly displayed under the selected device button. [Actual result]: - The list is not centered. See https://i.imgur.com/gYubpyo.png. [Regression range]: - This issue is not a regression. - Here is the pushlog https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=4037eb98974db1b1e0b5012c8a7f3a36428eaa11&tochange=3587b25bae302c1eed72968dbd7cef883e715948. [Additional notes]: - The issue is not reproducible if at step 5, the page is scrolled to the right until the device button hits the middle of the window. - Note that this issue is reproducible only if previously the '4K Ultra HD Television' device is selected.