[ui] Allow painting any content as background for component

GitOrigin-RevId: 96c9b865ed72b125bb2fef009514da4a44e84290

platform/platform-impl/api-dump-experimental.txt

@@ -245,6 +245,8 @@ f:com.intellij.openapi.vfs.newvfs.persistent.PersistentFSImpl
 - *s:setOfflineByDefault(com.intellij.openapi.vfs.VirtualFile,Z):V
 a:com.intellij.openapi.wm.impl.FrameTitleBuilder
 - *:getFileTitleAsync(com.intellij.openapi.project.Project,com.intellij.openapi.vfs.VirtualFile,kotlin.coroutines.Continuation):java.lang.Object
+f:com.intellij.openapi.wm.impl.IdeBackgroundUtil
+- *s:createTemporaryBackgroundTransform(javax.swing.JComponent,com.intellij.openapi.ui.Painter,com.intellij.openapi.Disposable):V
 f:com.intellij.openapi.wm.impl.content.ToolWindowContentUi
 - com.intellij.openapi.actionSystem.UiCompatibleDataProvider
 - com.intellij.ui.content.ContentUI

platform/platform-impl/src/com/intellij/ide/startup/impl/StartupManagerImpl.kt

@@ -215,9 +215,12 @@ open class StartupManagerImpl(private val project: Project, private val coroutin
           && pluginId.idString != "com.intellij.clion.plugin"
           && pluginId.idString != "com.jetbrains.codeWithMe"
           && pluginId.idString != "intellij.rider.plugins.cwm"
-          && pluginId.idString != "org.jetbrains.plugins.clion.radler") {
-        LOG.error("Only bundled plugin can define ${extensionPoint.name}: ${adapter.pluginDescriptor}")
-        continue
+          && pluginId.idString != "org.jetbrains.plugins.clion.radler"
+        ) {
+        if (!(pluginId.idString == "com.intellij.ml.llm" && adapter.assignableToClassName.endsWith("XNextRootPaneCustomizer"))) {
+          LOG.error("Only bundled plugin can define ${extensionPoint.name}: ${adapter.pluginDescriptor}")
+          continue
+        }
       }
 
       val activity = adapter.createInstance<InitProjectActivity>(project) ?: continue

platform/platform-impl/src/com/intellij/openapi/application/impl/InternalUICustomization.kt

@@ -1,6 +1,7 @@
 // Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.openapi.application.impl
 
+import com.intellij.openapi.Disposable
 import com.intellij.openapi.application.ApplicationManager
 import com.intellij.openapi.fileEditor.impl.EditorTabPainterAdapter
 import com.intellij.openapi.ui.Divider
@@ -49,6 +50,7 @@ open class InternalUICustomization {
 
   open fun createCustomDivider(isVertical: Boolean, splitter: Splittable): Divider? = null
 
+  open fun attachBackgroundGradient(component: JComponent, disposable: Disposable): Unit = Unit
 
   open fun getToolWindowsPaneThreeSplitterBackground(): Color = JBColor.GRAY
 

platform/platform-impl/src/com/intellij/openapi/wm/impl/IdeBackgroundUtil.java

@@ -14,6 +14,7 @@ import com.intellij.openapi.fileEditor.impl.EditorEmptyTextPainter;
 import com.intellij.openapi.fileEditor.impl.EditorsSplitters;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.ui.AbstractPainter;
+import com.intellij.openapi.ui.Painter;
 import com.intellij.openapi.util.Disposer;
 import com.intellij.openapi.util.Key;
 import com.intellij.openapi.util.text.Strings;
@@ -154,6 +155,39 @@ public final class IdeBackgroundUtil {
     createTemporaryBackgroundTransform(root, paintersHelper, disposable);
   }
 
+  /**
+   * Allows painting anything as a background for component and its children
+   */
+  @ApiStatus.Experimental
+  public static void createTemporaryBackgroundTransform(JComponent root,
+                                                        Painter painter,
+                                                        Disposable disposable) {
+    PainterHelper paintersHelper = new PainterHelper(root);
+    // In order not to expose MyGraphics class, we need to have a delegate that unwraps MyGraphics to Graphics2D
+    paintersHelper.addPainter(new Painter() {
+      @Override
+      public boolean needsRepaint() {
+        return painter.needsRepaint();
+      }
+
+      @Override
+      public void paint(Component component, Graphics2D g) {
+        painter.paint(component, MyGraphics.unwrap(g));
+      }
+
+      @Override
+      public void addListener(@NotNull Listener listener) {
+        painter.addListener(listener);
+      }
+
+      @Override
+      public void removeListener(Listener listener) {
+        painter.removeListener(listener);
+      }
+    }, root);
+    createTemporaryBackgroundTransform(root, paintersHelper, disposable);
+  }
+
   private static void createTemporaryBackgroundTransform(JComponent root, PainterHelper painterHelper, Disposable disposable) {
     Disposer.register(disposable, JBSwingUtilities.addGlobalCGTransform((c, g) -> {
       if (!UIUtil.isAncestor(root, c)) {