Blender Git Loki

Fix T89952: GPencil channel box selection offset

The channel box selection was offset for grease pencil layers.

This is a proposed fix by @yann-lty

Before:
{F10227973}

After:
{F10227974}

Reviewed By: #grease_pencil, antoniov

Maniphest Tasks: T89952

Differential Revision: https://developer.blender.org/D11962