I just came across this (again).
I was building a minimal ui without Controls (no javafx.controls), so
just using Panes and CSS.
When using a CSS variable from modena.css like -fx-text-base-color like so:
.pane {
-fx-background-color: -fx-text-base-color;
}
I suddenly got errors:
javafx.scene.CssStyleHelper calculateValue
WARNUNG: Caught 'java.lang.ClassCastException: class java.lang.String
cannot be cast to class javafx.scene.paint.Paint (java.lang.String is in
module java.base of loader 'bootstrap'; javafx.scene.paint.Paint is in
unnamed module of loader 'app')' while converting value for
'-fx-background-color' from rule '*.pane' in stylesheet file:/xxx
After a while, I found out that this is because of Control or
PopupControl will load modena.css - so If I don't have any, the CSS and
CSS variables are not available yet.
John provided more information - I think the user agent stylesheet
should be loaded on Application startup. Before the start(Stage) method
is called.
Any thoughts?
-- Marius
Am 28.11.2025 um 21:02 schrieb John Hendrikx:
I'm seeing something odd in Control. It tries to initialize the
default platform user agent stylesheet in a static initializer. This
makes any subclass of Control hard to test (as a unit) as it will try
to initialize the entire JavaFX platform.
I see little need for this. The stylesheet is loaded actually in 3
different places:
- Control
- PopupControl
- Application (if calling setUserAgentStylesheet with null)
Could we perhaps just defer this loading? Or perhaps load it when
another class is created that is required for CSS processing?
I could imagine the following options:
- Initialize this when Window or Scene class is loaded (required to
make use of CSS)
- Initialize this somewhere in Application as a default
- Instead of initializing the whole platform, set only a flag that
says "A Control class was loaded already, so when platform starts, set
the stylesheet" (if we're for some reason purposely only doing this
when a Control class is loaded...)
I have no idea why this is in Control or PopupControl specifically.
If I make an FX application without ever loading Control (using
Regions for example), should I be surprised that Modena styles are not
working when I put them on my regions? What if I do apply some style
(let's say "button"), should I be surprised the behavior changes when
I later do create (but not use) a Control as then suddenly Modena
styles start applying?
Is there some attempt here to prevent loading of the Modena stylesheet
when NOT using Controls?
The code in question in Control static initializer is this:
// Ensures that the default application user agent stylesheet is loaded
if(Application.getUserAgentStylesheet() == null) {
PlatformImpl.setDefaultPlatformUserAgentStylesheet();
}
I really see no reason why this code needs to be here specifically.
It in fact looks a bit of a hack.
--John
