[Markdown][IJPL-253568] Make the Markdown preview layout setting govern every editor

(cherry picked from commit 445bb72854646ef5db413fdf019402d1fe8f3fe8)

IJ-CR-220760

GitOrigin-RevId: 399603c2abd08b7ae62b59cdfa6d6c9c598a3798

plugins/markdown/core/src/org/intellij/plugins/markdown/ui/preview/MarkdownEditorWithPreview.java

@@ -6,9 +6,11 @@ import com.intellij.openapi.editor.Editor;
 import com.intellij.openapi.editor.event.VisibleAreaEvent;
 import com.intellij.openapi.editor.event.VisibleAreaListener;
 import com.intellij.openapi.editor.impl.EditorImpl;
+import com.intellij.openapi.fileEditor.FileEditorState;
 import com.intellij.openapi.fileEditor.TextEditor;
 import com.intellij.openapi.fileEditor.TextEditorWithPreview;
 import com.intellij.openapi.project.Project;
+import com.intellij.ui.JBSplitter;
 import org.intellij.plugins.markdown.MarkdownBundle;
 import org.intellij.plugins.markdown.settings.MarkdownSettings;
 import org.jetbrains.annotations.NotNull;
@@ -16,6 +18,7 @@ import org.jetbrains.annotations.NotNull;
 import java.awt.Point;
 
 public final class MarkdownEditorWithPreview extends TextEditorWithPreview {
+  private final MarkdownSettings settings;
   private boolean autoScrollPreview;
 
   /**
@@ -38,6 +41,8 @@ public final class MarkdownEditorWithPreview extends TextEditorWithPreview {
       !settings.isVerticalSplit()
     );
 
+    this.settings = settings;
+
     // allow launching actions while in preview mode;
     // FIXME: better solution IDEA-354102
     editor.getEditor().getContentComponent().putClientProperty(ActionUtil.ALLOW_ACTION_PERFORM_WHEN_HIDDEN, true);
@@ -64,6 +69,22 @@ public final class MarkdownEditorWithPreview extends TextEditorWithPreview {
     editor.getEditor().getScrollingModel().addVisibleAreaListener(new MyVisibleAreaListener(), this);
   }
 
+  @Override
+  protected @NotNull JBSplitter createSplitter() {
+    JBSplitter splitter = super.createSplitter();
+    // "Preview layout" must apply to a freshly created editor, not just to an already open one.
+    splitter.setOrientation(!settings.isVerticalSplit());
+    return splitter;
+  }
+
+  @Override
+  public void setState(@NotNull FileEditorState state) {
+    super.setState(state);
+    // "Preview layout" is a global default, so it must win over the per-file orientation
+    // that super.setState() restores from the editor state. See IJPL-253568.
+    handleLayoutChange(!settings.isVerticalSplit());
+  }
+
   @Override
   protected void onLayoutChange(Layout oldValue, Layout newValue) {
     super.onLayoutChange(oldValue, newValue);

plugins/markdown/test/src/org/intellij/plugins/markdown/ui/preview/MarkdownPreviewLayoutSettingTest.kt

@@ -0,0 +1,50 @@
+// Copyright 2000-2026 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package org.intellij.plugins.markdown.ui.preview
+
+import com.intellij.openapi.fileEditor.TextEditorWithPreview
+import com.intellij.openapi.fileEditor.impl.text.TextEditorProvider
+import com.intellij.openapi.util.Disposer
+import com.intellij.testFramework.fixtures.BasePlatformTestCase
+import com.intellij.ui.JBSplitter
+import com.intellij.util.ui.UIUtil
+import org.intellij.plugins.markdown.settings.MarkdownSettings
+
+/**
+ * see IJPL-253568
+ */
+class MarkdownPreviewLayoutSettingTest : BasePlatformTestCase() {
+  fun testSplitHorizontallySurvivesRestoredPerFileOrientation() {
+    val settings = MarkdownSettings.getInstance(project)
+    settings.update { it.isVerticalSplit = false }
+
+    val splitter = splitterOf(settings)
+    assertTrue("setting must be applied to a freshly created editor", splitter.orientation)
+  }
+
+  fun testRestoredStateDoesNotOverrideSetting() {
+    val settings = MarkdownSettings.getInstance(project)
+    settings.update { it.isVerticalSplit = false }
+
+    val editorWithPreview = createEditor(settings)
+    val splitter = UIUtil.findComponentOfType(editorWithPreview.component, JBSplitter::class.java)!!
+
+    editorWithPreview.setState(
+      TextEditorWithPreview.MyFileEditorState(TextEditorWithPreview.Layout.SHOW_EDITOR_AND_PREVIEW, null, null, false)
+    )
+
+    assertTrue("global setting must win over restored per-file orientation", splitter.orientation)
+  }
+
+  private fun splitterOf(settings: MarkdownSettings): JBSplitter =
+    UIUtil.findComponentOfType(createEditor(settings).component, JBSplitter::class.java)!!
+
+  private fun createEditor(settings: MarkdownSettings): MarkdownEditorWithPreview {
+    myFixture.configureByText("test.md", "# Heading\n\ntext\n")
+    val file = myFixture.file.virtualFile
+    val textEditor = TextEditorProvider.getInstance().getTextEditor(myFixture.editor)
+    val preview = MarkdownPreviewFileEditor(project, file, myFixture.editor.document)
+    val editorWithPreview = MarkdownEditorWithPreview(textEditor, preview, project, settings)
+    Disposer.register(testRootDisposable, editorWithPreview)
+    return editorWithPreview
+  }
+}