Component css gets reloaded with css-loader on any file change

[non25]

Hi dear devs and contributors.

I've noticed this behavior trying to reduce watch mode reload times when using tailwind+postcss through svelte-preprocess by monitoring with speed monitoring webpack plugin.

On any non-component change in the project with 20 components that uses in-component css there's 40 seconds of wait due to reloading of all Component.svelte.css in _virtualFiles.

I've been messing around with virtual.js for some hours trying to fix this.

Tried to pass fs.statSync(this.resourcePath)'s to use with virtual css file. Though that non-authentic VirtualFileStats triggered watcher to reload. Didn't work.

console.logged contents of _statStorage, _readFileStorage, _virtualFiles in compiler.inputFileSystem.purge and compiler.inputFileSystem._writeVirtualFile.
It appears that original purge doesn't clear any of that containers, commenting it's code doesn't change anything.
compiler.inputFileSystem.purge does trigger on any change.

svelte-loader/lib/virtual.js

Lines 15 to 30 in 929eebe

	var originalPurge = compiler.inputFileSystem.purge;
	compiler.inputFileSystem.purge = function() {
	if (originalPurge) {
	originalPurge.call(this, arguments);
	}
	if (this._virtualFiles) {
	Object.keys(this._virtualFiles).forEach(
	function(file) {
	var data = this._virtualFiles[file];
	setData(this._statStorage, file, [null, data.stats]);
	setData(this._readFileStorage, file, [null, data.contents]);
	}.bind(this)
	);
	}
	};

While we at it, what is the purpose of this?

svelte-loader/lib/virtual.js

Lines 40 to 49 in 929eebe

	const watchRunHook = (watcher, callback) => {
	this._watcher = watcher.compiler || watcher;
	callback();
	};
	if (compiler.hooks) {
	compiler.hooks.watchRun.tapAsync('VirtualModulesPlugin', watchRunHook);
	} else {
	compiler.plugin('watch-run', watchRunHook);
	}

Also there's an issue with similar task and problem, where author solved it by decorating NodeWatchFileSystem.
webpack/webpack#5824
Don't know if that is a good idea...

The main question is why watcher reloads virtual css files when they weren't changed...
Would like to get some insight into this.