This seems to be an issue with scaling, i.e. when the game is not shown at 100%, i.e. CTRL - and reset back CTRL+0
System Info: 1.135.644da79e70 (19.09.2018 11:04), scibuff (6869907), en9, en_US, WEB 7,1,1,0, Windows 10, Chrome/69.0.3497.100, 2133x1042, OpenGL Vendor=WebKit Version=WebGL 2.0 (OpenGL ES 3.0 Chromium) Renderer=WebKit WebGL GLSL=WebGL GLSL ES 3.00 (OpenGL ES GLSL ES 3.0 Chromium) (512 MB VRAM)