Seems to happen when I'm scrolling the list of threads using the mouse wheel. Random threads the pointer passes over open automatically on new background tabs. I can prevent it by pointing the mouse to the far left or right side of the page while rolling the mouse wheel. Other forums I visit never do this.

Does your mouse wheel have a switch under it that activates the current selection?

Using a ROG Strix Inpact III at the moment with a laptop. Also does it with an MX Master 3S, but less frequently. They both have on/off switches and pairing buttons on the bottom. The MX has a button below the mouse wheel but the ROG does not. Both can select a link by depressing the wheel instead of rolling it. Noticed that when a link is selected by clicking the wheel, the link is opened on a background tab with no change in focus, just like what is happening without clicking.

I think I found a workaround fix. Being that clicking the scroll wheel causes the same behavior this forum does on it's own while rolling the wheel, I went into the ROG mouse driver and disabled the scroll-click function. Weird behavior navigating this forum has now ceased with that mouse. Apparently the coding for this forum confuses the detection of scrolling and clicking the wheel. I don't know if there's a way of disabling it on a per-site basis so my workaround is global. I don't use scroll-click on other forums, so I won't miss it. If I do miss it in a Vegas FX function which involves pressing down on the wheel while dragging, I'll switch mice to that if necessary.