Fix widgets being styled with the wrong state on their first pass

`Ui::widget_style` reads the widget state from the response of the
previous pass. When there is no such response yet it fell back to
`WidgetState::Inactive`, styling the widget wrong for one pass.
Now we request a discard instead, so the widget is painted only once
we know its state.

Also make the default `SeparatorStyle` read `noninteractive.bg_stroke`
directly: a separator is never interactive, so its stroke should not
depend on the widget state. `inactive.bg_stroke` is `Stroke::NONE`,
which made the separator invisible.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/theme/default_style.rs

@@ -143,12 +143,12 @@ impl StyleProvider<LabelStyle> for DefaultStyle {
 
 impl StyleProvider<SeparatorStyle> for DefaultStyle {
     fn style(&mut self, modifiers: &StyleArgs<'_>) -> SeparatorStyle {
-        let StyleArgs { ctx, .. } = modifiers;
+        let StyleArgs { style, .. } = modifiers;
-        let ws: BaseStyle = ctx.get_widget_style(modifiers);
 
         SeparatorStyle {
             spacing: 6.0,
-            stroke: ws.frame.stroke,
+            // A separator is never interactive, so its stroke doesn't depend on the widget state:
+            stroke: style.visuals.widgets.noninteractive.bg_stroke,
         }
     }
 }

crates/egui/src/theme/mod.rs

@@ -8,7 +8,7 @@ pub use self::{style_provider::StyleProvider, themes::Themes};
 
 use crate::{
     Ui,
-    widget_style::{Classes, StyleArgs, WidgetStyle},
+    widget_style::{Classes, StyleArgs, WidgetState, WidgetStyle},
 };
 
 impl Ui {
@@ -19,11 +19,16 @@ impl Ui {
         id: crate::Id,
         classes: &Classes,
     ) -> S {
-        // Fetch the current state of the widget
+        // Fetch the state of the widget, as it was in the previous pass
-        let state = self
+        let state = if let Some(response) = self.read_response(id) {
-            .read_response(id)
+            response.widget_state()
-            .map(|r| r.widget_state())
+        } else {
-            .unwrap_or_default();
+            // We don't know the state of the widget yet, so we would style it wrong.
+            // Discard this pass and style it correctly in the next one.
+            self.ctx()
+                .request_discard("Widget style depends on a widget response we don't have yet");
+            WidgetState::default()
+        };
 
         self.get_widget_style::<S>(&StyleArgs {
             classes,