To first provide some context, I am running Windows with multiple monitors and LibreOffice is open on the left monitor. Now on to the issue, when hovering the mouse over a sidebar icon (with the sidebar closed, although I'm not sure that is relevant), the tooltip overflows accross to the other monitor, which results in a divided tooltip that is really difficult to read. Ideally, LibreOffice should recognize the edge of the monitor and adjust the tooltip to expand to the left. This expected behavior is indeed seen if LibreOffice is open on the right monitor.
Sorry, but this is not a bug. You as a multi-screen user have to manage your virtual desktop space and where you position LibreOffice. If LibreOffice is positioned full screen on the right monitor, against the physical/logical right screen edge--the tooltip is moved left of center to be fully readable, aligned right. And, if full screen against the physical/logical left screen edge of the left monitor (yes you can undock and dock the Sidebar on the left side) tooltips are moved right of center to be fully readable, aligned left. But, when you position LibreOffice on the right edge of the left monitor, you are saying that the logical desktop extends right onto the adjacent monitor, and that programs can and should use that space. LibreOffice positions the tooltip on the widget(s), at point of mouse-over/ selection, and the dialog will bleed over/split onto the next monitor since that is the available desktop. Although it can simply go off the edge if screens are asymmetrical, e.g. 1920x1200 left and 1200x1920 right. The left edge, right edge, top edge and bottom edge of the virtual desktop that MS Windows is managing are available to LibreOffice--and while Windows can set LibreOffice's frame full screen in one of the monitors--tool tips and dialog pop-ups are not going to be constrained down the middle.
Thanks for the explanation. While it may not technically be a bug, it is still a horrible user experience, but one I guess I will have to live with.