I recently changed my Chrome and Firefox to 'tabless' mode. Each tab gets its own Window. I did this because for work on Linux I have several different browsers. So what happens is I end up trying to 'alt tab' to get back to some web page, when it's actually buried in a hidden tab somewhere. So I have to alt-tab to switch to different browser windows, then hunt through all the tabs. This is slow and not good workflow. So I've been using all my browsers in 'tabless' mode.
Chrome and Firefox both have extensions for this. Vivaldi can work with all Chrome extensions so I added it there too. But I notice in Vivaldi when I close the tab/window with CTRL+W it pops back up a new window for some reason. The fix for this is go into Vivalid settings and do 3 things
1. Search for 'close tab' then scroll down to keyboard, remove the default close window shortcut by clicking the X to the right of the text box showing the keyboard shortcut (not visible until you hover).
3. Search again for 'close window' then scroll down to keyboard, remove the default close window shortcut by clicking the X (not visible until you hover).
3. Now for that shortcut change the 'close window' shortcut to CTRL+W by clicking in the text field, which is normally the close tab shortcut.
After this the problem of windows not going away is fixed.