I have a problem with Chromium displaying in fullscreen mode.
The webpage to be shown is made so to automatically adjust to the resolution of the screen.
I'll do better to explain with images.
This is the webpage i want to show:

This is obviously what I want to get:

But this is what actually happens with --kiosk:

The page is rendered according to the native resolution of the screen, but Chromium is placed as if the viewport has a 3 or 4px padding around the screen.
And this is what happens if, instead of using --kiosk, I use --app=<page_address>

The page is rendered according to the actual Chromium viewport, but it still has a padding, not taking full screen.
How do I get rid of that padding? Using Chromium in normal mode, with toolbars and other stuff, it works properly.
I'm using Arch Linux for Raspberry Pi 2 with just Xserver. With Raspbian, instead, Chromium works fine even in --kiosk mode, but there is also a Desktop Environment.