When --edge top there is a gap on the left/right side of the screen depending on the align value.

[Luqman]

This bug seems to have been fixed upstream. See https://github.com/dstu/trayer-srg/commit/9b0e69a192af68f132205c7dec5ed113b4393510

From there (and what I'm experiencing):

when I start trayer in xmonad with --SetPartialStrut true, xmonad does not properly avoid covering trayer. Instead, the following behavior is observed:

    If no --edge parameter is given, xmonad places windows on top of trayer.
    If --edge left is specified, behavior is as expected.
    If --edge right is specified, xmonad places windows on top of trayer.
    If --edge top is specified, xmonad places windows on top of trayer but acts as though trayer is present on the right side. (It leaves an empty band of space on the right.)
    If --edge bottom is specified, xmonad places windows on top of trayer.

It looks like there is a problem with the call made to XChangeProperty at panel.c:panel_set_wm_struct. Per XGetWindowProperty(3):

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Christopher Meiklejohn (cmeiklejohn)]

Confirmed; just started experiencing this behavior in debian testing a few days ago. Disabling trayer resolves the problem.

[Michael J. Holler (mjholler)]

Bug is more specifically within trayer's '--setPartialStrut' parameter. Setting it to false (had it previously set to true) solves this problem. Now I have a full screen instead of a 20 px gap on the right side. As long as you have something else to avoid the windows for you (xmobar, in my case) trayer won't be covered.