Add support for hot reload of CSS files added with `@StyleSheet`

[sissbruecker]

Describe your motivation

When using the legacy theme mechanism, I can modify any CSS file that is in the theme folder and get "hot reload", so a live update of the styles in the browser without having to refresh the page. This works both with or without Hotswap.

When adding a CSS file using @StyleSheet this does not work the same way:

• Without Hotswap, but when running mvn spring-boot:run, it reloads the full page when changing styles. This is better than nothing, but it's not hot reload.
• With Hotswap via the Vaadin IntelliJ plugin, nothing gets updated. I have to refresh the page manually after changing the styles.

In order to get developers to migrate from the legacy theme mechanism to using @StyleSheet we should ideally have feature parity between these two.

Describe the solution you'd like

Modifying a CSS file added with @StyleSheet should trigger hot reload, so that no page refresh is necessary. If hot reload is not feasible, Flow should at least force a page refresh.

Hot reload or page refresh should work the same way, regardless whether Hotswap is used or not.

One thing to keep in mind is that CSS files can import other CSS files using @import statements. All of those would have to be watched / updated.

[MarcinVaadin]

Current behaviour (alpha19):

Screen.Recording.2025-10-08.at.11.41.20.mov

Each blink is view reload. Notice that it requires multiple file saves for changes to be applied.

[sissbruecker]

Is that a Vaadin plugin feature or a Flow feature?

[MarcinVaadin]

Flow triggers update but Vaadin plugin does the recompile. So both.

[mcollovati]

About CSS files, since #22392, in dev mode the following folders are watched for changes

src/main/resources/META-INF/resources
src/main/resources/resources
src/main/resources/static
src/main/resources/public

[MarcinVaadin]

In IntelliJ, I need to recompile styles.css to see updates on running application. That's why changes are applied with a delay when you have plugin. Flow reloads view when files are not "recompiled" yet...

[MarcinVaadin]

Screen.Recording.2025-10-08.at.12.03.04.mov

[mshabarov]

Notice that it requires multiple file saves for changes to be applied.

This is what I see as well. A timing issue somewhere in between file save/compile and page refresh.
Sometimes my changes are visible right away, sometimes after few other change, e.g. by adding a space and CMD+S/Ctrl+S. If I stop by a breakpoint here, then page reload works each time, so definitely a timing issue.

Another observation is that even if I recompile the file, sometimes my latest changes appear on the page, but sometimes it suddenly picks and renders one of the previous changes.

And ofc it's a bug that it reloads the full page.