fix: make class attribute overwrite behavior consistent between SSR and CSR (closes #4248) (#4439)

Fixes #4248

During SSR, multiple `class` attributes were incorrectly concatenating
instead of overwriting like they do in browsers. This inconsistency
caused code that appeared to work in SSR to fail in CSR/hydration.

The fix distinguishes between two types of class attributes:
- `class="..."` attributes should overwrite (clear previous values)
- `class:name=value` directives should merge (append to existing classes)

Implementation:
- Added `should_overwrite()` method to `IntoClass` trait (defaults to `false`)
- Modified `Class::to_html()` to clear buffer before rendering if `should_overwrite()` returns `true`
- Implemented `should_overwrite() -> true` for string types (`&str`, `String`, `Cow<'_, str>`, `Arc<str>`)
- Tuple type `(&'static str, bool)` keeps default `false` for merge behavior

Added comprehensive tests to verify:
- `class="foo" class:bar=true` produces `"foo bar"` (merge)
- `class:foo=true` works standalone
- Correct behavior with macro attribute sorting
- Global class application

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

leptos/tests/ssr.rs

@@ -103,6 +103,76 @@ fn test_classes() {
     assert_eq!(rendered.to_html(), "<div class=\"my big  red car\"></div>");
 }
 
+#[cfg(feature = "ssr")]
+#[test]
+fn test_class_with_class_directive_merge() {
+    use leptos::prelude::*;
+
+    // class= followed by class: should merge
+    let rendered: View<HtmlElement<_, _, _>> = view! {
+        <div class="foo" class:bar=true></div>
+    };
+
+    assert_eq!(rendered.to_html(), "<div class=\"foo bar\"></div>");
+}
+
+#[cfg(feature = "ssr")]
+#[test]
+fn test_solo_class_directive() {
+    use leptos::prelude::*;
+
+    // Solo class: directive should work without class attribute
+    let rendered: View<HtmlElement<_, _, _>> = view! {
+        <div class:foo=true></div>
+    };
+
+    assert_eq!(rendered.to_html(), "<div class=\"foo\"></div>");
+}
+
+#[cfg(feature = "ssr")]
+#[test]
+fn test_class_directive_with_static_class() {
+    use leptos::prelude::*;
+
+    // class:foo comes after class= due to macro sorting
+    // The class= clears buffer, then class:foo appends
+    let rendered: View<HtmlElement<_, _, _>> = view! {
+        <div class:foo=true class="bar"></div>
+    };
+
+    // After macro sorting: class="bar" class:foo=true
+    // Expected: "bar foo"
+    assert_eq!(rendered.to_html(), "<div class=\"bar foo\"></div>");
+}
+
+#[cfg(feature = "ssr")]
+#[test]
+fn test_global_class_applied() {
+    use leptos::prelude::*;
+
+    // Test that a global class is properly applied
+    let rendered: View<HtmlElement<_, _, _>> = view! { class="global",
+        <div></div>
+    };
+
+    assert_eq!(rendered.to_html(), "<div class=\"global\"></div>");
+}
+
+#[cfg(feature = "ssr")]
+#[test]
+fn test_multiple_class_attributes_overwrite() {
+    use leptos::prelude::*;
+
+    // When multiple class attributes are applied, the last one should win (browser behavior)
+    // This simulates what happens when attributes are combined programmatically
+    let el = leptos::html::div().class("first").class("second");
+
+    let html = el.to_html();
+
+    // The second class attribute should overwrite the first
+    assert_eq!(html, "<div class=\"second\"></div>");
+}
+
 #[cfg(feature = "ssr")]
 #[test]
 fn ssr_with_styles() {

tachys/src/html/class.rs

@@ -57,6 +57,10 @@ where
         _style: &mut String,
         _inner_html: &mut String,
     ) {
+        // If this is a class="..." attribute (not class:name=value), clear previous value
+        if self.class.should_overwrite() {
+            class.clear();
+        }
         class.push(' ');
         self.class.to_html(class);
     }
@@ -156,6 +160,12 @@ pub trait IntoClass: Send {
     /// Renders the class to HTML.
     fn to_html(self, class: &mut String);
 
+    /// Whether this class attribute should overwrite previous class values.
+    /// Returns `true` for `class="..."` attributes, `false` for `class:name=value` directives.
+    fn should_overwrite(&self) -> bool {
+        false
+    }
+
     /// Renders the class to HTML for a `<template>`.
     #[allow(unused)] // it's used with `nightly` feature
     fn to_template(class: &mut String) {}
@@ -289,6 +299,10 @@ impl IntoClass for &str {
         class.push_str(self);
     }
 
+    fn should_overwrite(&self) -> bool {
+        true
+    }
+
     fn hydrate<const FROM_SERVER: bool>(
         self,
         el: &crate::renderer::types::Element,
@@ -346,6 +360,10 @@ impl IntoClass for Cow<'_, str> {
         IntoClass::to_html(&*self, class);
     }
 
+    fn should_overwrite(&self) -> bool {
+        true
+    }
+
     fn hydrate<const FROM_SERVER: bool>(
         self,
         el: &crate::renderer::types::Element,
@@ -403,6 +421,10 @@ impl IntoClass for String {
         IntoClass::to_html(self.as_str(), class);
     }
 
+    fn should_overwrite(&self) -> bool {
+        true
+    }
+
     fn hydrate<const FROM_SERVER: bool>(
         self,
         el: &crate::renderer::types::Element,
@@ -460,6 +482,10 @@ impl IntoClass for Arc<str> {
         IntoClass::to_html(self.as_ref(), class);
     }
 
+    fn should_overwrite(&self) -> bool {
+        true
+    }
+
     fn hydrate<const FROM_SERVER: bool>(
         self,
         el: &crate::renderer::types::Element,