Interferes with Proof General overlay marker

[heptadecagon]

Expected behavior

When submitting a line to the proof assistant, a small triangle (the "overlay marker") appears in the left margin after the line to show it's been submitted.

Actual behavior

After enabling centered-window-mode, the triangle is replaced with =>. It covers the first two characters of the line.

Steps to reproduce

Install Proof General and a proof assistant, like Coq, and enable centered-window-mode. Next, open a *.v file, like those in the Unimath library:

https://github.com/UniMath/UniMath

or the Software Foundations textbook:

https://softwarefoundations.cis.upenn.edu/

With the file open, press C-c C-n to submit the first line. The symbol => should be visible.

Backtraces if necessary (M-x toggle-debug-on-error)

Versions of centered-window-mode, Emacs, OS etc.

• centered-window-mode: 1.4.0
• Emacs: 25.3.1 (Railway Emacs, GNU Emacs for Mac OS X)
• OS: MacOS 10.13.3 High Sierra