Fixed readyStateCompleteListeners never flushing when initialized after DOM ready - #15571

Unfortunately this will have negative effect on websites that load slowly (or when the internet connection is slow), as it postpones certain mechanisms until the state is complete (all style sheets and images finished loading). Particularly, this check is used to remove the fallback style, which is inserted to prevent white flashes.

Is your SVG dynamically generated? Dark Reader is supposed to watch for DOM and inline style changes, but maybe there's a bug. Also this SVG analysis is not perfect, maybe switching it off completely will work better for you. E.g. by supplying a "fix" argument like {ignoreImageAnalysis: ['svg *'], invert: ['svg']}. We also still have experimental theme property {immediateModify: true} that disables the ready state chack.

Nothing gets postponed, because the change removes no case from the guard, it only adds one. The line decides whether the listener gets registered, nothing else:

if (!isReadyStateComplete()) {          // was !isDOMReady()
    const onReadyStateChange = () => { /* unchanged */ };
    document.addEventListener('readystatechange', onReadyStateChange);
}

and it's negated, so the narrower predicate registers in more cases, not fewer:

export let isDOMReady: () => boolean = () => {
    return document.readyState === 'complete' || document.readyState === 'interactive';
};

export function isReadyStateComplete(): boolean {
    return document.readyState === 'complete';
}

!isDOMReady() is true only at loading. !isReadyStateComplete() is true at loading and at interactive. Every case that registered before still registers, plus one that never did. So no mechanism can fire later than it used to, the fallback style included.

That added case is init at interactive, which is what a deferred script gets. There the listener was never registered, so readyStateCompleteListeners never flushed at all. The SVG fill overrides weren't running late, they weren't running.

The SVG is static markup. overrideInlineStyle queues the fill handling itself because the heuristic needs getBoundingClientRect(), so the watchers can't cover it.