OK, I got it! I searched for style.height in the script, and it turned out that there is an assignment cellp.style.height = cellp.clientWidth to all board squares in Display(). I think this was an attempt to force the squares to remain square when the browser window starts to compress the Diagram in the horizontal direction. But it appears the way you handle the square sizes causes the clientWidth to become 0px, while the assignment erases your setting of the height in vh units.
After commenting out this assignment from betzaFlex.js the Diagram now also shows up when the coordinates are not displayed. Newer browsers might be more clever in calculating the clientWidth.
OK, I got it! I searched for style.height in the script, and it turned out that there is an assignment cellp.style.height = cellp.clientWidth to all board squares in Display(). I think this was an attempt to force the squares to remain square when the browser window starts to compress the Diagram in the horizontal direction. But it appears the way you handle the square sizes causes the clientWidth to become 0px, while the assignment erases your setting of the height in vh units.
After commenting out this assignment from betzaFlex.js the Diagram now also shows up when the coordinates are not displayed. Newer browsers might be more clever in calculating the clientWidth.