CesiumWidget has zero size and the globe is never created

• DeveloperError: Expected width to be greater than 0, actual value was 0 at CesiumWidget.draw and Viewer.resize.
• viewer.canvas.width is 0.
• Zoom handles and credits pile up in the top-left corner.

1. Print el.clientWidth and el.clientHeight before construction.
2. Walk up the ancestors logging each clientHeight to find the first zero.
3. Check for display none on any ancestor, such as an unopened tab, drawer or modal.
4. Check flex parents, where flex 1 without min-height 0 collapses to zero.
5. Check whether the script runs before DOMContentLoaded, before styles are applied.

Give the container a definite height chain with html and body at 100%, and cesiumContainer at flex 1, min-height 0 and position relative. Initialise only when the container is visible, waiting on a ResizeObserver before new Cesium.Viewer, and call viewer.resize on later size changes.

DeveloperError: Expected width to be greater than 0, actual value was 0
    at CesiumWidget.draw (CesiumWidget.js:1042)
    at Viewer.resize (Viewer.js:1301)

var el = document.getElementById('cesiumContainer');
console.log(el.clientWidth, el.clientHeight);       // 0 0

var p = el;
while (p && p !== document.body) {
  console.log(p.tagName, p.className, p.clientHeight);
  p = p.parentElement;
}

• Creating the Viewer while hidden and calling resize afterwards, when internal buffers were already sized at 0 and some versions keep a broken canvas.
• Setting height 100% on the container without giving any ancestor a height, so the percentage resolves against auto.