Bug 293476

Created attachment 475351 [details] Reproduction code index.html Under specific circumstances, slotted content does not render in a custom element. It seems that a repaint makes the content appear again. See the attached index.html file. When opening in Safari on either macOS or iOS, you will see a button "Make element appear". This is not the only thing that is supposed to be rendered on the page: there should also be a custom element with slotted text "Hello world". However, this custom element is not visible. Clicking the button will set the custom element's color CSS to red, making the custom element finally appear. As mentioned, this is under specific circumstances. The index.html file contains comments that further elaborate on this. For example: * The custom element appears to require wrapping elements (an inline element such as a span, and a block element such as a div). * There must be another custom element on the page that turns itself into a block element with CSS after a timeout. The circumstances are specific, especially related to the timeout. But when custom elements implement some form of lazy loading, this scenario is very likely to occur. macOS, Sequoia 15.5 (Apple M3) * Safari 18.5 – NOK * Chrome 137 – OK * Firefox 138 – OK iPhone 15 Pro Max, iOS 18.4.1 * Safari – NOK * Brave 1.78 (103) – NOK