Add change files for FormatSpecHandle fix

Author: hl662

common/changes/@itwin/core-frontend/nam-format-spec-handle-fix-spec_2026-05-05-19-55.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Fix FormatSpecHandle stale state during onFormattingReady",
+      "type": "none",
+      "packageName": "@itwin/core-frontend"
+    }
+  ],
+  "packageName": "@itwin/core-frontend",
+  "email": "50554904+hl662@users.noreply.github.com"
+}

common/changes/@itwin/core-quantity/nam-format-spec-handle-fix-spec_2026-05-05-19-55.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Fix FormatSpecHandle stale state during onFormattingReady",
+      "type": "none",
+      "packageName": "@itwin/core-quantity"
+    }
+  ],
+  "packageName": "@itwin/core-quantity",
+  "email": "50554904+hl662@users.noreply.github.com"
+}

core/frontend/src/quantity-formatting/QuantityFormatter.ts

@@ -1403,12 +1403,19 @@ export class QuantityFormatter implements UnitsProvider, FormattingSpecProvider
         formatProps,
         formatName: name,
       });
-      if (!this._formatSpecsRegistry.has(name))
-        this._formatSpecsRegistry.set(name, new Map());
-      const unitMap = this._formatSpecsRegistry.get(name)!;
-      if (!unitMap.has(persistenceUnitName))
-        unitMap.set(persistenceUnitName, new Map());
-      unitMap.get(persistenceUnitName)!.set(effectiveSystem, { formatterSpec, parserSpec });
+      let unitMap = this._formatSpecsRegistry.get(name);
+      if (!unitMap) {
+        unitMap = new Map();
+        this._formatSpecsRegistry.set(name, unitMap);
+      }
+
+      let systemMap = unitMap.get(persistenceUnitName);
+      if (!systemMap) {
+        systemMap = new Map();
+        unitMap.set(persistenceUnitName, systemMap);
+      }
+
+      systemMap.set(effectiveSystem, { formatterSpec, parserSpec });
     } else {
       throw new Error(`Unable to find format properties for ${name} with persistence unit ${persistenceUnitName}`);
     }

core/frontend/src/test/QuantityFormatter.test.ts

@@ -1133,13 +1133,17 @@ describe("Multi-system KoQ registry", () => {
     await qf.addFormattingSpecsToRegistry({ name: "TestKoQ.LENGTH", persistenceUnitName: "Units.M", formatProps: simpleDecimalFormat, system: "imperial" });
     await qf.addFormattingSpecsToRegistry({ name: "TestKoQ.LENGTH", persistenceUnitName: "Units.M", formatProps: simpleDecimalFormat, system: "metric" });
 
-    let handle: ReturnType<typeof qf.getFormatSpecHandle>;
-    const observedSpecs: FormatterSpec[] = [];
-    qf.onFormattingReady.addListener(() => {
-      observedSpecs.push(handle.formatterSpec!);
+    const handleRef: { current: ReturnType<typeof qf.getFormatSpecHandle> | undefined } = { current: undefined };
+    const observedSpecs: Array<FormatterSpec | undefined> = [];
+    const removeReadyListener = qf.onFormattingReady.addListener(() => {
+      if (!handleRef.current)
+        throw new Error("Expected FormatSpecHandle to be created before onFormattingReady fired");
+
+      observedSpecs.push(handleRef.current.formatterSpec);
     });
 
-    handle = qf.getFormatSpecHandle("TestKoQ.LENGTH", "Units.M");
+    const handle = qf.getFormatSpecHandle("TestKoQ.LENGTH", "Units.M");
+    handleRef.current = handle;
     const imperialFormatterSpec = qf.getSpecsByNameAndUnit({
       name: "TestKoQ.LENGTH",
       persistenceUnitName: "Units.M",
@@ -1160,6 +1164,8 @@ describe("Multi-system KoQ registry", () => {
     expect(observedSpecs).toHaveLength(1);
     expect(observedSpecs[0]).toBe(metricFormatterSpec);
     expect(observedSpecs[0]).not.toBe(imperialFormatterSpec);
+
+    removeReadyListener();
     handle[Symbol.dispose]();
   });
 });