Created attachment 8791072 [details] Test case described above User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:48.0) Gecko/20100101 Firefox/48.0 Build ID: 20160823121617 Steps to reproduce: Firefox 48.0.2, Windows 7 Ultimate x64, monitor size 1920x1080 Viewport set to client width and scale 1, simple CSS reset on *, html and body set to 100% width and 100% height, body min-width set to 320px and overflow-x set to auto. Also set a media query max-width=319px to change the body background color from default red to green when width is < 320px. Actual results: No matter how narrow I resize the window, the horizontal scrollbar does not appear. When inspecting the body element in Firebug or checking in Firefox console, the reported document.body.clientWidth never drops below 401px, even though the actual client width (window size) is less than 401px. This also appears to affect media queries as the background does not change color. Expected results: Once the window is resized below 320px, a horizontal scrollbar should appear, and the body background should change from red to green. Works fine in Chrome, Firefox not.
Upon restart of Firefox, this seems to have rectified itself. Which makes bug hunting all the more difficult as it's apparently an intermittent problem.