Whole window flickers when css hover used on macOS

When moving the cursor over part of a webview page that has :hover styling, it can cause the entire window to flicker. This does not happen when using 6.3.2, or when using 6.4.1 on other platforms.

To reproduce, load up the markdowneditor Qt example, and replace the single main window in main.cpp with 2:

MainWindow window1;
window1.show();
MainWindow window2;
window2.show();

Then paste the following in each window:

<style>div {transition-property: background; transition-duration: 0.5s;} div:hover { background: #eee; height: 5em; }</style>

<div>hello</div>

It happens even without animating the transition, but that seemed to exacerbate the problem. The level of flicker seems to vary - sometimes it happens every few seconds, and sometimes it happens multiple times in one movement.