This is a problem which I've been seeing for a couple of months across a range of eeschema revisions, but I didn't report it because I assumed it was likely to just go away if I ignored it. :-)
The problem is that lines which are perfectly horizontal or vertical in the page layout editor (see one of the attached images) don't necessarily draw as perfectly horizontal or vertical in eeschema (see the other attached image).
The screenshots are from the following project (though as far as I can tell it happens on all of my projects): https://github.com/virtualagc/virtualagc/tree/schematics/Schematics/2005938C
Application: Eeschema
Version: 6.0.0-unknown-ef03afe~86~ubuntu16.04.1, release build
Libraries:
wxWidgets 3.0.2
libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.11 libidn/1.32 librtmp/2.3
Platform: Linux 4.4.0-53-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
Boost: 1.58.0
OpenCASCADE Community Edition: 6.8.0
Curl: 7.47.0
Compiler: GCC 5.4.0 with C++ ABI 1009
Build settings:
KICAD_SCRIPTING=ON
KICAD_SCRIPTING_MODULES=ON
KICAD_SCRIPTING_PYTHON3=OFF
KICAD_SCRIPTING_WXPYTHON=ON
KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
KICAD_SCRIPTING_ACTION_MENU=ON
BUILD_GITHUB_PLUGIN=ON
KICAD_USE_OCE=ON
KICAD_USE_OCC=OFF
KICAD_SPICE=ON
On a Retina display this shows as the line changing thickness so I had assumed this had something to do with the shaders. But maybe it's just that the line isn't 100% straight?