fix(linter/jsx-a11y/mouse-events-have-key-events): handle nullish event handlers (#27306)

`mouse-events-have-key-events` reports absent hover handlers such as
`onMouseOver={undefined}` and misses active hover handlers paired with
`onFocus={null}`. The same issue affects hover-out/blur and configured
handler lists.

Use one local predicate to skip nullish hover values, reject nullish
keyboard values, and continue to a later configured active hover
handler. Preserve existing component, spread and bare-attribute
behavior. Bare attributes retain Oxc's prior behavior, which differs
from [the upstream
rule](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/v6.10.2/src/rules/mouse-events-have-key-events.js).

Validation:
- The new regression test demonstrated five false-positive cases and
four false-negative cases before the fix.
- Both focused rule tests pass, with 22 new regression/control cases
including default/configured handlers, ordering, dynamic functions,
components and bare attributes.
- Independent AI code review found no blocking issues; suggested
bare-attribute coverage was added.
- Full post-commit `just ready` passed: 98 test targets, 5,061 passed /
161 ignored / 0 failed; formatting, code generation, checks, Clippy and
documentation completed with a clean worktree.

Fixes #27305

AI-assisted with Codex.

---------

Co-authored-by: Pybsama <294532706+Pybsama@users.noreply.github.com>
Co-authored-by: Cameron Clark <cameron.clark@hey.com>

crates/oxc_linter/src/rules/jsx_a11y/mouse_events_have_key_events.rs

@@ -1,4 +1,7 @@
-use oxc_ast::{AstKind, ast::JSXAttributeValue};
+use oxc_ast::{
+    AstKind,
+    ast::{Expression, JSXAttributeItem, JSXAttributeValue},
+};
 use oxc_diagnostics::OxcDiagnostic;
 use oxc_macros::declare_oxc_lint;
 use oxc_span::{GetSpan, Span};
@@ -96,22 +99,12 @@ impl Rule for MouseEventsHaveKeyEvents {
 
         for handler in &self.0.hover_in_handlers {
             if let Some(jsx_attr) = has_jsx_prop(jsx_opening_el, handler) {
-                if get_prop_value(jsx_attr).is_none() {
+                if !has_handler_value(jsx_attr) {
                     continue;
                 }
 
-                match has_jsx_prop(jsx_opening_el, "onFocus").and_then(get_prop_value) {
-                    Some(JSXAttributeValue::ExpressionContainer(container)) => {
-                        if let Some(expr) = container.expression.as_expression()
-                            && expr.is_undefined()
-                        {
-                            ctx.diagnostic(miss_on_focus(jsx_attr.span(), handler));
-                        }
-                    }
-                    None => {
-                        ctx.diagnostic(miss_on_focus(jsx_attr.span(), handler));
-                    }
-                    _ => {}
+                if !has_jsx_prop(jsx_opening_el, "onFocus").is_some_and(has_handler_value) {
+                    ctx.diagnostic(miss_on_focus(jsx_attr.span(), handler));
                 }
 
                 break;
@@ -120,20 +113,12 @@ impl Rule for MouseEventsHaveKeyEvents {
 
         for handler in &self.0.hover_out_handlers {
             if let Some(jsx_attr) = has_jsx_prop(jsx_opening_el, handler) {
-                if get_prop_value(jsx_attr).is_none() {
+                if !has_handler_value(jsx_attr) {
                     continue;
                 }
 
-                match has_jsx_prop(jsx_opening_el, "onBlur").and_then(get_prop_value) {
-                    Some(JSXAttributeValue::ExpressionContainer(container))
-                        if container.expression.is_undefined() =>
-                    {
-                        ctx.diagnostic(miss_on_blur(jsx_attr.span(), handler));
-                    }
-                    None => {
-                        ctx.diagnostic(miss_on_blur(jsx_attr.span(), handler));
-                    }
-                    _ => {}
+                if !has_jsx_prop(jsx_opening_el, "onBlur").is_some_and(has_handler_value) {
+                    ctx.diagnostic(miss_on_blur(jsx_attr.span(), handler));
                 }
 
                 break;
@@ -142,10 +127,29 @@ impl Rule for MouseEventsHaveKeyEvents {
     }
 }
 
+fn has_handler_value(attribute: &JSXAttributeItem<'_>) -> bool {
+    match get_prop_value(attribute) {
+        Some(JSXAttributeValue::ExpressionContainer(container)) => {
+            let Some(expression) = container.expression.as_expression() else {
+                return true;
+            };
+            let expression = expression.get_inner_expression();
+            !expression.is_undefined() && !matches!(expression, Expression::NullLiteral(_))
+        }
+        Some(_) => true,
+        None => false,
+    }
+}
+
 #[test]
 fn test() {
     use crate::tester::Tester;
 
+    let configured = Some(serde_json::json!([{
+        "hoverInHandlers": ["onMouseOver", "onMouseEnter"],
+        "hoverOutHandlers": ["onMouseOut", "onMouseLeave"]
+    }]));
+
     let pass = vec![
         ("<div onMouseOver={() => void 0} onFocus={() => void 0} />;", None),
         ("<div onMouseOver={() => void 0} onFocus={() => void 0} {...props} />;", None),
@@ -197,6 +201,21 @@ fn test() {
             "<div onMouseLeave={() => {}} />",
             Some(serde_json::json!([{ "hoverOutHandlers": ["onPointerLeave"] }])),
         ),
+        ("<div onMouseOver={undefined} />", None),
+        ("<div onMouseOut={undefined} />", None),
+        ("<div onMouseOver={null} />", None),
+        ("<div onMouseOut={null} />", None),
+        ("<div onMouseOver={undefined} onMouseOut={null} />", None),
+        ("<div onMouseOver />", None),
+        ("<div onMouseOut />", None),
+        ("<div onMouseOver={handler} onFocus={handler} />", None),
+        ("<div onMouseOut={handler} onBlur={handler} />", None),
+        ("<div onMouseOver={null} onMouseEnter={handler} onFocus={handler} />", configured.clone()),
+        (
+            "<div onMouseOut={undefined} onMouseLeave={handler} onBlur={handler} />",
+            configured.clone(),
+        ),
+        ("<Custom onMouseOver={handler} onFocus={null} />", None),
     ];
 
     let fail = vec![
@@ -234,6 +253,16 @@ fn test() {
             "<div onPointerLeave={() => {}} />",
             Some(serde_json::json!([{ "hoverOutHandlers": ["onPointerLeave"] }])),
         ),
+        ("<div onMouseOver={handler} onFocus={null} />", None),
+        ("<div onMouseOut={handler} onBlur={null} />", None),
+        ("<div onMouseOver={handler} onFocus={undefined} />", None),
+        ("<div onMouseOut={handler} onBlur={undefined} />", None),
+        ("<div onMouseOver={handler} onFocus />", None),
+        ("<div onMouseOut={handler} onBlur />", None),
+        ("<div onMouseOver={null} onMouseEnter={handler} />", configured.clone()),
+        ("<div onMouseOut={undefined} onMouseLeave={handler} />", configured.clone()),
+        ("<div onMouseOver={null} onMouseEnter={handler} onFocus={null} />", configured.clone()),
+        ("<div onMouseOut={undefined} onMouseLeave={handler} onBlur={null} />", configured),
     ];
 
     Tester::new(MouseEventsHaveKeyEvents::NAME, MouseEventsHaveKeyEvents::PLUGIN, pass, fail)

crates/oxc_linter/src/snapshots/jsx_a11y_mouse_events_have_key_events.snap

@@ -99,3 +99,73 @@ source: crates/oxc_linter/src/tester.rs
    ·      ─────────────────────────
    ╰────
   help: Try to add `onBlur`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOver` must be accompanied by `onFocus` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOver={handler} onFocus={null} />
+   ·      ─────────────────────
+   ╰────
+  help: Try to add `onFocus`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOut` must be accompanied by `onBlur` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOut={handler} onBlur={null} />
+   ·      ────────────────────
+   ╰────
+  help: Try to add `onBlur`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOver` must be accompanied by `onFocus` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOver={handler} onFocus={undefined} />
+   ·      ─────────────────────
+   ╰────
+  help: Try to add `onFocus`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOut` must be accompanied by `onBlur` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOut={handler} onBlur={undefined} />
+   ·      ────────────────────
+   ╰────
+  help: Try to add `onBlur`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOver` must be accompanied by `onFocus` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOver={handler} onFocus />
+   ·      ─────────────────────
+   ╰────
+  help: Try to add `onFocus`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseOut` must be accompanied by `onBlur` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:6]
+ 1 │ <div onMouseOut={handler} onBlur />
+   ·      ────────────────────
+   ╰────
+  help: Try to add `onBlur`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseEnter` must be accompanied by `onFocus` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:25]
+ 1 │ <div onMouseOver={null} onMouseEnter={handler} />
+   ·                         ──────────────────────
+   ╰────
+  help: Try to add `onFocus`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseLeave` must be accompanied by `onBlur` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:29]
+ 1 │ <div onMouseOut={undefined} onMouseLeave={handler} />
+   ·                             ──────────────────────
+   ╰────
+  help: Try to add `onBlur`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseEnter` must be accompanied by `onFocus` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:25]
+ 1 │ <div onMouseOver={null} onMouseEnter={handler} onFocus={null} />
+   ·                         ──────────────────────
+   ╰────
+  help: Try to add `onFocus`.
+
+  ⚠ jsx-a11y(mouse-events-have-key-events): `onMouseLeave` must be accompanied by `onBlur` for accessibility.
+   ╭─[mouse_events_have_key_events.tsx:1:29]
+ 1 │ <div onMouseOut={undefined} onMouseLeave={handler} onBlur={null} />
+   ·                             ──────────────────────
+   ╰────
+  help: Try to add `onBlur`.