I use Xephyr to make high resolution screenshots of a web interface. Screenshots must look good when printed. So I run 

$ Xephyr :108 -dpi 300 -screen 3413x1920

(This resolution equals 1024×576 @90 dpi multiplied by 3.33(3), which is the ratio between 300 and 90 dpi)

but without passing -resizeable the window is smaller than it should be, and a part of the browser in fullscreen is hidden. I am not talking about those parts, that are initially hidden, because Xephyr window is way too large, I’m talking about those you won’t see even if you grab the window and drag to its very right or bottom edges. 
I first noticed that at home, where I have 1920×1080 display, but I thought that the resolution is fixed to certain values. With the resolution I used, 3413×1920, which — note that — smaller than 2×1920 × 2×1080, I didn’t experience any troubles. But when I took my laptop and tried to open the same Xephyr window on a 1366×768 screen, I’ve seen only a part! 

Strange is, that -resizeable key fixes the issue. Though I have to ‘untile’ the window first this way, which is not convenient to do each time and requires a setting in the WM to do this automatically.

Steps to reproduce:
1. Choose a screen resolution at least 2.5 times larger than yours.
2. Pick a high dpi, 300 for instance.
3. Run 
   $ Xephyr :108 -dpi 300 -screen XRESxYRES
4. Run some application.
   $ DISPLAY=:108 vivaldi
   Make it fullsreen. Window -> Go fullscreen or F11 works for vivaldi.
5. See the window cropped.
6. Drag the window so its right edge would be at physical screen edge. Move the mouse to the left edge of the physical screen, press and hold the left mouse button on the Xephyr window, so it could be dragged. Drag the cursor all the way to the right edge of the screen.
7. See that Xephyr window is limited to two widths of your screen.