fix: exitBeforeEnter in useTransition doesn't work when used with trail (#1868) (#2407)

Co-authored-by: Jon Hieb <hiebj@wildidea.io>

Author: hiebj

.changeset/gentle-tomatoes-nail.md

@@ -0,0 +1,5 @@
+---
+'@react-spring/core': patch
+---
+
+fix: exitBeforeEnter in useTransition doesn't work when used with trail

packages/core/src/hooks/useTransition.test.tsx

@@ -195,6 +195,36 @@ describe('useTransition', () => {
 
     expect(rendered).toEqual([1])
   })
+
+  it('should work with both exitBeforeEnter and trail together', async () => {
+    const props = {
+      from: { t: 0 },
+      enter: { t: 1 },
+      leave: { t: 0 },
+      exitBeforeEnter: true,
+      trail: 100,
+    }
+
+    // Start with two items
+    update([0, 1], props)
+    expect(rendered).toEqual([0, 1])
+
+    global.mockRaf.step()
+
+    // Replace with two new items - the old ones should leave first with trail
+    update([2, 3], props)
+
+    global.mockRaf.step()
+
+    // Old items should still be visible (leaving with trail)
+    expect(rendered).toEqual([0, 1])
+
+    // Wait for all leave animations to complete
+    await global.advanceUntilIdle()
+
+    // New items should now be visible
+    expect(rendered).toEqual([2, 3])
+  })
 })
 
 let result: RenderResult | undefined

packages/core/src/hooks/useTransition.tsx

@@ -307,6 +307,9 @@ export function useTransition(
       if (t.ctrl.idle) {
         const idle = transitions.every(t => t.ctrl.idle)
         if (t.phase == TransitionPhase.LEAVE) {
+          // Remove this transition from exitingTransitions as soon as it completes.
+          exitingTransitions.current.delete(t)
+
           const expiry = callProp(expires, t.item)
           if (expiry !== false) {
             const expiryMs = expiry === true ? 0 : expiry
@@ -323,12 +326,6 @@ export function useTransition(
         }
         // Force update once idle and expired items exist.
         if (idle && transitions.some(t => t.expired)) {
-          /**
-           * Remove the exited transition from the list
-           * this may not exist but we'll try anyway.
-           */
-          exitingTransitions.current.delete(t)
-
           if (exitBeforeEnter) {
             /**
              * If we have exitBeforeEnter == true