Fix checkbox DOM sync (#50991)

# Fix checkbox DOM sync

Unbreaks checkboxes with custom value attributes.

## Description

There was a trivial logic error in the DOM syncing code, causing the `value` attribute on checkboxes to get wrongly overwritten in a certain case (if there was a custom value attribute and `checked` was unchanged).

Fixes #50995

## Customer Impact

In some cases, checkbox values would get corrupted after an enhanced navigation. This would break form functionality.

The specific scenario was "checkboxes with a custom value attribute, when an enhanced nav leaves the `checked` state unchanged".

## Regression?

- [ ] Yes
- [x] No

[If yes, specify the version the behavior has regressed from]

## Risk

- [ ] High
- [ ] Medium
- [x] Low

Very low because it's a tiny code edit, and only affects a very specific scenario, and the old logic was (in retrospect) definitely incorrect.

## Verification

- [x] Manual (required)
- [x] Automated

## Packaging changes reviewed?

- [ ] Yes
- [ ] No
- [x] N/A

Author: SteveSandersonMS

src/Components/Web.JS/dist/Release/blazor.server.js

@@ -354,8 +354,10 @@ function ensureEditableValueSynchronized(destination: Element, value: any) {
   } else if (destination instanceof HTMLSelectElement && destination.selectedIndex !== value) {
     destination.selectedIndex = value as number;
   } else if (destination instanceof HTMLInputElement) {
-    if (destination.type === 'checkbox' && destination.checked !== value) {
-      destination.checked = value as boolean;
+    if (destination.type === 'checkbox') {
+      if (destination.checked !== value) {
+        destination.checked = value as boolean;
+      }
     } else if (destination.value !== value) {
       destination.value = value as string;
     }

src/Components/Web.JS/dist/Release/blazor.web.js

@@ -469,6 +469,22 @@ describe('DomSync', () => {
     expect(selectElem.selectedIndex).toBe(2);
   });
 
+  test('should handle checkboxes with value attribute', () => {
+    // Checkboxes require even more special-case handling because their 'value' attribute
+    // has to be handled as a regular attribute, and 'checked' must be handled similarly
+    // to 'value' on other inputs
+
+    const destination = makeExistingContent(`<input type='checkbox' value='first' checked />`);
+    const newContent = makeNewContent(`<input type='checkbox' value='second' checked />`);
+
+    const checkboxElem = destination.startExclusive.nextSibling as HTMLInputElement;
+
+    // Act/Assert
+    synchronizeDomContent(destination, newContent);
+    expect(checkboxElem.checked).toBeTruthy();
+    expect(checkboxElem.value).toBe('second');
+  });
+
   test('should treat doctype nodes as unchanged', () => {
     // Can't update a doctype after the document is created, nor is there a use case for doing so
     // We just have to skip them, as it would be an error to try removing or inserting them