Incorrect WebGL rendering on specific scenes

A user reported a bug for Konqueror bug 489195 stating that some WebGL scenes in web page https://www.ibiblio.org/e-notes/webgl/webgl.htm weren't rendering correctly. I tried viewing the same scenes using other browsers based on QtWebEngine 6 (Falkon, QuteBrowser and the simple browser example included in QtWebEngine itself) and the results were the same. I think this means it is a problem with QtWebEngine itself. I tried browsers not based on QtWebEngine(Chromium, Firefox and Vivaldi) and they rendered the scenes correctly.

The scenes not rendered correctly in the given page are two:

• NaCl: moving the mouse to rotate the image makes the balls disappear, while they should remain visible
• Lorenz model: in the black square there's only a white dot moving, while there should also be several colored lines, as shown in the image on the left of the link in the original page.