fix: pinned md preview switches when opening another file

When md viewer is pinned and user switches to another md or HTML file,
the preview would switch instead of staying pinned. Two fixes:
- _loadMdviewrPreview: check urlPinned before activating MarkdownSync
  on a new document when md viewer is already active
- _activeDocChanged: skip openLivePreview when md viewer is pinned to
  prevent HTML LP iframe from replacing the pinned md viewer

Update pin test to cover md→HTML and md→md switches with negative
assertions verifying the pinned preview stays visible.

Author: abose

src/extensionsIntegrated/Phoenix-live-preview/main.js

@@ -876,6 +876,10 @@ define(function (require, exports, module) {
         _setTitle(relativeOrFullPath, currentPreviewFile, "");
 
         if (_isMdviewrActive) {
+            if (urlPinned && !force) {
+                // Pinned — don't switch the md viewer content
+                return;
+            }
             // Mdviewr iframe already loaded, just update the sync for the new document
             MarkdownSync.activate(currentDoc, $iframe, baseURL);
             return;
@@ -1160,6 +1164,10 @@ define(function (require, exports, module) {
     }
 
     function _activeDocChanged(event, focusedEditor, lostEditor) {
+        // When md viewer is pinned, don't open HTML live preview — keep the md viewer visible
+        if (_isMdviewrActive && urlPinned) {
+            return;
+        }
         if(!LivePreviewSettings.isUsingCustomServer() && !LiveDevelopment.isActive()
             && (panel.isVisible() || StaticServer.hasActiveLivePreviews())) {
             // we do this only once after project switch if live preview for a doc is not active.

test/spec/LiveDevelopmentMultiBrowser-test.js

@@ -1764,20 +1764,36 @@ define(function (require, exports, module) {
 
             await waitsForLiveDevelopmentToOpen();
             await awaitsForDone(SpecRunnerUtils.openProjectFiles([`readme.md`]),
-                "SpecRunnerUtils.openProjectFiles simple1.html");
+                "SpecRunnerUtils.openProjectFiles readme.md");
 
             await _waitForIframeSrc(`readme.md`);
             let pinURLBtn = testWindow.$(testWindow.document.getElementById("pinURLButton"));
             pinURLBtn.click();
 
+            // Pin active on readme.md — switch to SVG, md viewer should stay
             await awaitsForDone(SpecRunnerUtils.openProjectFiles([SVG_IMAGE_PATH]),
                 SVG_IMAGE_PATH);
-            await awaits(500);
             await _waitForIframeSrc(`readme.md`);
 
-            pinURLBtn.click();
+            // Switch to HTML file while md is pinned
+            await awaitsForDone(SpecRunnerUtils.openProjectFiles(["simple1.html"]),
+                "open simple1.html while md pinned");
+            // Negative assertion: wait for async switch to settle, then verify it did NOT happen
+            await awaits(500);
+            let mdIFrame = _getMdPreviewIFrame();
+            expect(mdIFrame && mdIFrame.style.display !== "none").toBeTrue();
 
-            await _waitForIframeSrc(SVG_IMAGE_PATH);
+            // Switch to another md file while md is pinned
+            await awaitsForDone(SpecRunnerUtils.openProjectFiles(["sample.md"]),
+                "open sample.md while md pinned");
+            // Negative assertion: wait for async switch to settle, then verify it did NOT happen
+            await awaits(500);
+            mdIFrame = _getMdPreviewIFrame();
+            expect(mdIFrame && mdIFrame.style.display !== "none").toBeTrue();
+
+            // Unpin — should switch to current file (sample.md)
+            pinURLBtn.click();
+            await _waitForIframeSrc(`sample.md`);
 
             await endPreviewSession();
         }, 30000);