fix(compiler): preserve &ngsp; between sibling control flow blocks

`findConnectedBlocks` scans siblings after an `@if` to collect connected
`@else`/`@else if` blocks. Whitespace-only text nodes encountered during
the scan were eagerly added to `processedNodes`, marking them as "do not
emit", before confirming whether a connected block actually followed.

By the time `findConnectedBlocks` runs, `WhitespaceVisitor` has already
converted `&ngsp;` (and ` `) into a plain space character, making
them indistinguishable from insignificant whitespace via `.trim().length`.
When the next sibling was a second, unrelated `@if` instead of `@else`,
the scan stopped but the text node was already silently dropped.

Fix by deferring the `processedNodes` insertion into a pending buffer and
only committing those nodes once a connected block is confirmed to follow.

Fixes #55791

packages/compiler/src/render3/r3_template_transform.ts

@@ -217,9 +217,7 @@ class HtmlAstToIvyAst implements html.Visitor {
         attrs.bound,
         boundEvents,
         directives,
-        [
-          /* no template attributes */
-        ],
+        [/* no template attributes */],
         children,
         references,
         variables,
@@ -560,6 +558,11 @@ class HtmlAstToIvyAst implements html.Visitor {
     predicate: (blockName: string) => boolean,
   ): html.Block[] {
     const relatedBlocks: html.Block[] = [];
+    // Whitespace-only text nodes to mark as processed only if a connected block follows them.
+    // We defer this so that significant whitespace (e.g. &ngsp; or  , both of which look
+    // like insignificant whitespace by the time we get here) between two unrelated @if blocks
+    // is not silently eaten.
+    const pendingTextNodes: html.Text[] = [];
 
     for (let i = primaryBlockIndex + 1; i < siblings.length; i++) {
       const node = siblings[i];
@@ -569,11 +572,10 @@ class HtmlAstToIvyAst implements html.Visitor {
         continue;
       }
 
-      // Ignore empty text nodes between blocks.
       if (node instanceof html.Text && node.value.trim().length === 0) {
-        // Add the text node to the processed nodes since we don't want
-        // it to be generated between the connected nodes.
-        this.processedNodes.add(node);
+        // Collect whitespace-only text nodes; only mark them as processed once we confirm
+        // they precede a connected block (e.g. @else / @else if).
+        pendingTextNodes.push(node);
         continue;
       }
 
@@ -582,6 +584,12 @@ class HtmlAstToIvyAst implements html.Visitor {
         break;
       }
 
+      // A connected block was found — commit the pending whitespace nodes as processed.
+      for (const pending of pendingTextNodes) {
+        this.processedNodes.add(pending);
+      }
+      pendingTextNodes.length = 0;
+
       relatedBlocks.push(node);
       this.processedNodes.add(node);
     }

packages/compiler/test/render3/r3_template_transform_spec.ts

@@ -2636,6 +2636,58 @@ describe('R3 template transform', () => {
       ]);
     });
 
+    it('should preserve &ngsp; between two sibling @if blocks', () => {
+      expectFromHtml(
+        `@if (true) {<span>Hello</span>}&ngsp;@if (true) {<span>World</span>}`,
+      ).toEqual([
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'Hello'],
+        ['Text', ' '],
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'World'],
+      ]);
+    });
+
+    it('should preserve plain whitespace between two sibling @if blocks when preserveWhitespaces is enabled', () => {
+      expectFromR3Nodes(
+        parse(`@if (true) {<span>Hello</span>}\n@if (true) {<span>World</span>}`, {
+          preserveWhitespaces: true,
+        }).nodes,
+      ).toEqual([
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'Hello'],
+        ['Text', '\n'],
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'World'],
+      ]);
+    });
+
+    it('should preserve &nbsp; between two sibling @if blocks when preserveWhitespaces is enabled', () => {
+      expectFromR3Nodes(
+        parse(`@if (true) {<span>Hello</span>}&nbsp;@if (true) {<span>World</span>}`, {
+          preserveWhitespaces: true,
+        }).nodes,
+      ).toEqual([
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'Hello'],
+        ['Text', '\u00A0'],
+        ['IfBlock'],
+        ['IfBlockBranch', 'true'],
+        ['Element', 'span'],
+        ['Text', 'World'],
+      ]);
+    });
+
     describe('validations', () => {
       it('should report an if block without a condition', () => {
         expect(() =>