No vertical scroll bar ...

No vertical scroll bar ...

https://fr.surveymonkey.com/mp/legal/cookies/

Works ok with Google Chrome.

Again you will say that the problem is at te webmaster of the site :-)

https://validator.w3.org/ reports > 20 warnings and > 100 errors.

But SeaMonkey 2.49.4 shows vertical scroll bare properly.> User agent: 
Mozilla/5.0 (X11; Linux i686; rv:52.0) Gecko/20100101 Firefox/52.0 
SeaMonkey/2.49.4 > Build identifier: 20180711182954

But Google Chrome can cope with it :-)

My: User agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) 
Gecko/20100101 Firefox/60.0 SeaMonkey/2.53.3

AND I notice that the page load with the vertical scroll-bar who vanished  by going to the right until it is outside the window, so inaccessible :-)

I'm see the same in my SeaMonkey 2.53.3 on Linux.

The Page Down/Page Up and arrow keys still work, but the mouse scrollwheel doesn't.

I don't have any extensions, so trying safe mode probably won't help.

I noticed a difference between the site in SM and Firefox.

The page heading in SeaMonkey "Polituque relative aux cookies" is all on one line. In Firefox it is two lines with "cookies" on the second line.

--
OS: Ubuntu Linux 18.04LTS - Gnome Desktop
https://www.thunderbird.net/en-US/get-involved/
https://give.thunderbird.net/en-US/

Well, yes. The "body" stanza of the site CSS includes the rule:

     overflow:  -moz-scrollbars-none;

If I disable that rule using Developer Tools the scrollbar re-appears.

I have no idea what they are trying to accomplish with that.

Lem Johnson

Maybe this is to passively "force" users not to use SeaMonkey. Survey Monkey may be wanting to reduce the number of browsers they have to test their web page with so they make the web page display incorrectly for any other browser than Chrome (chrome variations also) or Firefox. The hope is that users will switch to either a Chrome variation or Firefox. 

This tactic is not something I thought a professional company would use to reduce the number of browsers they need to test with their site.

I had been thinking it looked fine for me, with SeaMonkey 2.53.3 on Linux Mint MATE. Then just realised that NoScript was blocking scripts from running. Allowing the scripts leads to the scrollbar disappearing. So seems to be something in their scripting that leads to the scrollbar being removed.

Firefox 80.0 indicates that "-moz-scrollbars-none" is an invalid value for the "overflow" property. So presumably it works on Firefox because the newer version has dropped support for that Mozilla-specific value. Probably a case of SurveyMonkey having left that bit of CSS in following some other change, checking that it works OK on the latest Firefox, and not noticing the effect of the left-over CSS on older versions which still recognise it.

You /might/ get somewhere reporting to SurveyMonkey that the latest Firefox indicates that this is an invalid value (and by the way, its presence happens to break the page on some other browsers they don't support, so getting rid of the invisible error in Firefox would also help users of those other browsers). But they probably don't care about underlying errors anyway as long as it looks OK on the limited set of browsers they test with, and almost certainly don't care if it doesn't work on "unsupported" browsers.

-- 
Mark.