[BUGFIX] @model becomes undefined or changes to the wrong route's model during Glimmer component willDestroy

[johanrd]

Summary

• Fixes a long-standing issue Route @model argument value timing issue #18987 where @model becomes undefined (or changes to the wrong route's model) during Glimmer component willDestroy when transitioning between routes
• Adds a controller identity check alongside the existing lastState === state guard in the outlet helper's model compute ref
• Adds a smoke test in the scenario test suite (based on Add a test that verifies @model is stable between route transitions #20959 by @Windvis)

Details

The @model reference in route templates is built from a chain of compute refs that read from the shared outlet state tree via dynamic scope. When transitioning, _setOutlets() rebuilds this tree, and all refs in the chain immediately see the new state.

The existing guard (lastState === state) freezes the model value when the outlet changes at the same level. But when a parent outlet is torn down first, the child outlet's outer compute ref never re-evaluates, so lastState is never updated. The guard passes, andmodelRef reads from the wrong outlet state.

The fix compares the current outlet's controller identity against the expected one. Since each route has a unique controller singleton, this detects when the outletRef has been redirected — even when lastState hasn't been updated yet.

From git history, the bug may have existed since @model was introduced in v3.14.0 (16b74a5). The original implementation read directly from the outletRef chain with no guard at all. The lastState === state guard added in v4.0.0 (commit 7d334cf) partially mitigated same-level transitions but did not catch parent-level teardown.

Fixes #18987
Supersedes #20959

Test plan

• Smoke test in smoke-tests/scenarios/basic-test.ts: transitions through sibling, parent, cousin, and unrelated routes, verifying this.args.model in willDestroy Based on Add a test that verifies @model is stable between route transitions #20959 but moved to smoke-test to avoid the vite.config import of glimmer-component
• Full internal test suite passes (5907/5907)
• Scenario tests pass for both v1 (classic/embroider-webpack) and v2 (embroider-vite) app builds

Before the fix, the added smoke tests fail with:

not ok 1 - Acceptance | @model stability during route transitions: @model should be stable when transitioning out of the route                                                                                                                                                                                                            
      actual:   b,b,d,e,
      expected: b,b,b,b,b
      message:  The @model value should remain stable in willDestroy for all transition types

ember-source	Result	Notes
workspace:* (with fix)	PASS	Local build includes the bugfix
~6..11.0	FAIL	Last version
~5.12.0	FAIL	Last 5.x
~4.0.0	FAIL	First 4.x
~3.28.0	FAIL	Last 3.x

Cowritten by claude