**Solved** Bug: Closing tabs with middle click with 3x monitors in span mode.
First of all, I'm very, very happy that we have a spiritual successor to the Opera of old! Liking very much what I see so far, I've been spreading the good word to my friends. Now, to the issue at hand. I'm fond of using middle click for opening links and closing tabs. Opening links in background with middle click works flawlessly. Closing tabs with middle click, not so much. On my home pc, I'm running three monitors in span mode (nVidia Surround) at 5760x1200 - Vivaldi running on the middle screen. When I middle click on a tab to close it, the tab closes - but also drops the browser out of maximized mode, and moves the whole browser window about half a screen to the right - so I got half the browser screen on the middle monitor, and the other half on the right monitor. This is indeed troubling behavior. Now, if I run Vivaldi on the rightmost monitor, and close a tab with middle click, the browser window is promptly moved to the leftmost monitor. Browser ping pong. Repeated closing of tabs with middle click marches the browser window across the monitors, until it stops with half the browser screen on middle monitor, and half on the right. Other browsers work fine. Also running Vivaldi at work, with two monitors in extended mode. Works as intended. Reconfigured my home setup to run in extended mode, worked fine. Also works fine on single screen. Tested with both nVidia GeForce 344.65 and 347.52. Win7 64-bit I7-5930K, 32GB Ram 2x GTX 970 Sli 3x Dell U2412m
Just for the record, it seems like this is a bug related to DisplayFusion, not Vivaldi.
Fixed it by adding compatibility settings for Vivaldi, namely "Disable middle-click window moving"