How do I switch between fullscreen without losing quality

I'm using Browser object in my game to switch between fullscreen and windowed mode. If I set the mode in "request fullscreen" to stretch (letterbox scale), I can enter fullscreen without any problems. But when I switch back to windowed mode, my game's position is off, and part of it isn't on the screen. The only solution I've been able to find is set the mode to Centered, and set "fullscreen in browser" to letterbox scale. But if I do that, my 800x600 game looks blurred. This is very important in my case, since I use pixel art graphics. Do you know any solution to this problem? If not, I am probably going to report the blurring bug and hope to get it fixed.

Update: I think I just got why the screen was blurred using letterbox scale. The NW.js window is bigger than 800x600, which is the game's size. I don't understand why though.

I really need to fix this problem, since my game should look well in windowed mode and be able to switch to fullscreen. Does anyone have an idea of what's going on?