I have worked with side panels popping up at an edge event for ages. With an update to 16.04, by top panel doesn't autohide, even if set so. The same panel does, when moved to another display side. My clear guess is that this has to make with the second screen: I have a smaller, second, screen on top of the first screen, and the panel and its auto-hide work everywhere except on that edge, which is at the same time the lower edge of the smaller panel above. Actually, under 14.04, sometimes the auto-hide panel on that edge wouldn't want to show; though now it shows all the time.
I observed the same behaviour, more or less. Scenario: I have a side (left) auto-hide panel I often change configuration from one monitor to two monitors. Let's suppose I'm on a single-monitor configuration and that everything is ok. Now, turn off the laptop, connect the second monitor, located on the right of the laptop. When kde starts, the panel is still located on the left side of leftmost display (the laptop one), that is ok, but is not hidden, that is not ok. Simply hovering on the panel make it disappear and from now on it works fine. plasma 5.9.0 apps 16.12.1 frameworks 5.30.0 QT 5.7.1 (gentoo)
Yup, this is a side-effect of that bug. When I move the panel from the wrong desktop to the correct one, what was auto-hide' is not auto-hide any longer. This is quite lousy: Seemingly nobody has thought of storing - and retrieving - the proper location and settings during a display change. Why do I say that? Because there *is* a setting 'primary display', only the panel does not follow this setting. I filed a bug report for that, which received a 'works for me I think, on 5.8'. I leave it for someone on 5.8 to check, and please check both: - following the primary display - keeping the properties as they were saved
Happens to me too on an up to date Neon install (Plasma 5.9.2+KDE Frameworks 5.31.0+Qt 5.7.1). Also on an up to date Kubuntu+backports (but that's just proof the bug has been there for a while). The description of this bug is very accurate: the panel hides as expected as long as it's not on the edge shared with another monitor. Can this at least be moved out of "unconfirmed" now?
Happens to me too.
Looks related to https://bugs.kde.org/show_bug.cgi?id=351175
*** This bug has been marked as a duplicate of bug 351175 ***