>>> My Info: Win7_64, Nightly 52, 32bit, ID 20161028030204 (2016-10-28) STR_1: 0. Set DPI -> 125% : A) Set DPI level -> 125% in your OS [OR] B) Set layout.css.devPixelsPerPx -> 1.25 1. Open url [1] 2. Click on the button 3. In the opened window, click on <select> 4. Hover mouse over the bottom pixel in select, move mouse to the left and right AR: Unwanted scrolling happens in drop-down list of options ER: List of options should stay still > [1] data:text/html,<button onclick='S="data:text/html,<select>";for(i=0;i<100;i++)S+="<option>Option "+i;window.open(S,"","height=400");'>Open@ Nobody; OK to take it and work on it: It seems that this is a regresion caused by your change. Please have a look.

User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 This issue can be reproduced on the Fx build mentioned by you, using the steps from comment 0. On latest Nightly (Build ID: 20170104030214), I cannot reproduce it. I did a Find Fix range using mozregression and reached this pushlog: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=aea479acaeb8ed3ac0f48235ff349042b82a61ba&tochange=a2059d25a0aec35af01993f1f2dd68144921e94e The fix made for bug 1128311 has fixed also this issue. Closing this as WFM. If anyone can still reproduce it on latest versions, feel free to reopen the issue and provide more information.

There's something wrong in the way you tested Nightly 2017-01-04. The regression range you provided contains irrelevant changes, and it looks as if you reproduced the but on each version except for the latest one. Have you forgot to set HiDPI -> 125%? Please re-test.

There is nothing wrong in the way I've tested Latest Nightly. I've used the same steps for all the Nightly versions tested. I've set the layout.css.devPixelsPerPx to 1.25 and set the DPI level to 125% in the OS and I cannot reproduce the issue.

>>> My Info: Win7_64, Nightly 53, 32bit, ID 20170104030214 (2017-01-04) Reproducible. If you haven't tested Nightly 2017-01-04 after comment 2, please test. Otherwise it's unexplainably; Simply remove NI, but change status back to "Unconfirmed" based on this comment and comment 2.

Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 Mozilla/5.0 (Windows NT 6.3; WOW64; rv:53.0) Gecko/20100101 Firefox/53.0 This issue is no longer reproducible on the Nightly x64 build starting with Build ID: 20170104030214. Find fix range is in comment 1. On the latest Nightly (Build ID: 20170108030212) x86 the issue is still reproducible. Thus, changing the status and assigning a component.