Tooltips for all icons in the "Notification area" bahave the same - bluetooth, wifi, clipman, power manager, etc. Other panel plugins such as PulseAudio, Keyboard layout, Clock work correctly.
Behaviour is still present in:
xfce4-panel-gtk3 4.13.4-2
What I observed:
It seems to be dependent on the pixel size of the panel. Below 35 pixels hight everything is fine. Above 34 Pixels tooltips are shown underneath the icons. If I change the icon size of the notification area to a value over 39px (which is ridiculously large) the toolstips are shown above the icons. The problem is also not present if I tick "reserve space", but of course this is not a solution since maximized windows disappear behind the panel. The values are probably not consistent across systems. For me the problem persists after trying several themes.
I have the same problem, starting with panel size 30px and higher. I thought that they are completely missing, but now I have noticed that they are under icon and hard to notice.
I'm seeing with two bottom, horizontal, non-overlapping panels. Panel 1 has systray, panel 2 is reserving space around the borders. When the height of panel 1 is less than or equal to the height of panel 2, the tooltips disappear (I can't tell if they are being displayed somewhere else). If I then increase the height of panel 1 to larger than the height of panel 2, or increase the size of the icons in the systray, the tooltips appear again. When panel 2 is not reserving space, this problem does not happen at all. I had network-manager-applet, xfce4-power-manager and remmina in the systray.
This bug does not seem to be notification-area-exclusively, I encounter this very issue on the entire panel. So if I want tooltips to work, I have to put the panel to the top side of the screen, not so great.
A way to work around this issue is setting "Don't reserve space on borders" to "YES", then the tooltips appear appropriately. But this option is often not desirable, too.
I use only a single panel. When placed at the bottom of the screen, the generic monitor tooltip is also shown outside de screen (as well as the workspace switcher tooltips if hovering starts from its bottom, so as mentioned it affects all widgets).
I have found the following hints which may help in diagnostics:
If the panel height is 48 or less pixels the tooltips are properly placed (I used 50 to cause the tasklist having 2 rows) at least in my 1200 pixels height screen.
I found a hack to properly display the tooltip without setting "Don't reserve space on borders" to "YES". However, the hack causes that setting, despite not apparently enabled, to be actually enabled (even if you cycle it to ON and later again to OFF): this is proven by the fact that the maximized windows disappear behind the panel (which seems until now the only known way to "fix" the bug). The hack, for anybody interested on it (just in case this behaviour could be considered a bug by itself) consists in (1) open in the settings editor the xfce4-panel settings and write down the "y" coordinate of the "position"; (2) manually drag the (unlocked) panel a bit up; (3) edit the y coordinate in the settings to restore the original value (the panel moves back to the initial position, and no longer honours the "Don't reserve space on borders" setting until it is manually moved again).
So, in summary: when the "Don't reserve space on borders" setting is enabled (either explicitly or by a unexpected behaviour) the tooltips are properly shown. Otherwise they are displayed outside the screen, which in the case of a generic monitor with a huge tooltip (e.g. https://lightful.github.io/xfce-hkmon/) becomes a catastrophe. Only if the panel is at the bottom of the screen (when placed at the top it seems that always are properly placed, no matter if hovering starts from the top or from the bottom of the panel).
As a provisional workaround for my personal use case, I have set the panel height to 48 instead of 50 pixels and added the following snippet to PATH/gtk.css to continue having two rows in the tasklist (one of the features I love from XFCE!):
If hovering starts and ends just AT THE TOP of the panel (the first pixel or maybe the first couple of pixels) the tooltip is properly displayed above the panel, even when the panel height is beyond 48 pixels.
However, it takes too much time to carefully hover just at the edge of the panel...
The trick of using a panel of 48 or less pixels height is not always enough. Yes, most of the tooltips are now shown inside the screen, but there is an exception: the tooltip from the mixer. A difference I see compared to the other panel items is that it doesn't expands across the full panel height...
I think this is a toolkit problem, I have tried to investigate it but the panel doesn't really do anything special about tooltip placement.
Trying to hook into tooltip placement is not straightforward and at a certain point you would need to touch internal APIs that Gtk+ doesn't export.
So to be honest I'm afraid this cannot be resolved at the time being (to my knowledge).
I have a similar problem but I also bring some insight. The tooltips on my bottom panel (used to launch applications) appear out of screen or partially out of screen.tooltip-2020-11-19_00.29.34.mkv
An interesting fact is that the unfortunate behaviour only happens when "Automatically hide the panel" is set to "Never".
This still seems to be an issue with the latest versions of xfce-base/xfce4-panel-4.16.2 and x11-libs/gtk+-3.24.26 on Gentoo.
Besides the quirk found by @fa-bien earlier, I found out the following on my system:
Tooltips are only incorrectly displayed (i.e., outside the screen) with plugins which are marked "external" by Panel Preferences, but not all. In my case the "Status Tray Plugin" (which by the way is provided by the xfce4-panel package, so no clue why it's marked as "external"...) and "Action Buttons" display the tooltip incorrectly, while the "external" plugins "Battery Monitor" and "PulseAudio Plugin" and all other ("internal") Panel items are displaying the tooltips correctly.
When enabling "Don't reserve space on borders" in the Panel Preferences, everything is displayed correctly, as stated earlier.
When increasing the Status Tray icons to 30 pixels or above, tooltips are shown correctly again.
Okay here is a workaround I ended up using. Use Don't reserve space on borders to make the panel "floating" and then go to Workspaces -> Margins and set the appropriate margins the same as the panel size (so maximized windows don't go behind the panel)
This bug (which I think is the same one as reported here and here) also exists in Debian 11 (bullseye), which has 4.16.2. My bottom panel is set to 100% width, 50 pixel height (the minimum to allow a double-row window switcher).
Here is an example involving the clipman plugin:
One anomaly I've noticed involves the Window Buttons plugin - the bottom row of buttons has the tooltip appearing below the mouse (and therefore off-screen), but the upper row places the tooltip more sensibly:
Edited to add:
I can confirm that setting Don't reserve space on borders helps me; I already had a bottom Workspace margin set (93px, because I have two panels at the bottom of my screen), and that alone wasn't sufficient.