Please use this code to reproduce. This code will load a webpage with animation so that Qt keeps refreshing.
Both normal html element update or webgl canvas refresh will cause this.

Screen record: 2024-03-12 10-15-08.mp4
The animation is smooth at the beginning. After playing for some time, it suddenly becomes lagging and random pixels appear. Stable reproduce. Win10+NVIDIA.

This code loads THREE.js example to keep the webengine refreshing. Other workflow can be: keep hovering mouse over different html elements (hover effect) and cause the webengine to refresh continuously.

#include <QtWidgets/QApplication>
#include <QtWidgets/QWidget>
#include <QtWebEngineWidgets/QWebEngineView>

int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	// tlw: red.
	QWidget tlw;
	tlw.resize(1920, 1080);
	tlw.setStyleSheet("QWidget { background-color: red; }");

	// webengine, d3d11, keep refreshing
	QWebEngineView webengine(&tlw);
	webengine.setGeometry(0, 0, 1920, 1080);
	webengine.setUrl(QUrl("https://threejs.org/examples/#webgl_animation_skinning_additive_blending"));

	tlw.show();
	return app.exec();
}