Steps to reproduce:
1. Download and open attached project "774745_GLLines.zip"
2. Switch render path to Deferred
3. Build project including "Test" scene and play it

Actual result: Notice that there is a white line drawn by GL.Lines. Resize the window. Notice that the line disappears

Expected result: Line does not disappear after re-sizing the window

Possible workaround: It does not happen with "Forward" rendering path

Tested and reproduced on:
Windows 10
Unity 5.1.4f1, 5.2.4p1, 5.3.3f1, 5.4.0b8