Earlier I clicked on the wrong link because a forum button moved just when the scrollbar disappeared (the page is re-centered).
Most websites enumerate the same repair solutions, which of course don't apply here (reset config, disable extensions, reinstall or update Chrome, etc). A few mention the experimental "Overlay scrollbars" in chrome://flags/ but it didn't change anything for me (you may try, one never knows).
I tried to change the theme but it doesn't change anything.
I only have this on the forums/ pages and subsites (like usercp, search, …). Never on the main page, never on the database or the CRPG analyzer, staff members, … (checking with a small window to make sure there is a need for a scrollbar, of course).
I tried to compare the content of the forum page source, but there's too much for me to analyze.
Maybe this
Code:
<body id="bodychange";overflow-y: scroll;>
(or code in one of the included .js)
The scrollbar only disappears some time after the page is displayed, so it looks like something triggered with a delay, once everything is loaded, or something like that.