feat: render streaming thinking blocks in chat turns

Renders the model's thinking deltas in a collapsible banner above each assistant turn (Copilot and subagent), backed by the LSP plumbing added in the previous PR.

- ThinkingBlock: collapsible streaming banner (spinner -> generated title / cancelled), markdown body parsed into titled sections.
- ThinkingSection: gutter dot + bold title + markdown body for each parsed section.
- ThinkingTurnWidget: extends BaseTurnWidget, accumulates Thinking deltas, seals on first reply/agent round, fires thinking/generateTitle, and finalizes on cancel.
- BaseTurnWidget: onChatMessageCancelled hook (default no-op) so subclasses can react to cancel events.
- CopilotTurnWidget / SubagentTurnWidget / SubagentMessageBlock: re-parented onto ThinkingTurnWidget.
- ChatContentViewer: routes thinking deltas to the active turn, seals before reply/agent rounds, seals at WorkDoneProgressKind.end.
- ChatView: don't drop progress reports that carry only thinking content.
- Messages / messages.properties: NLS keys for the in-progress / cancelled / expand / collapse strings.
- dark.css / light.css: shared secondary-text color rule for the new ThinkingBlock selector.

Author: ethanyhou

com.microsoft.copilot.eclipse.ui/css/dark.css

@@ -115,14 +115,10 @@
 	background-color: #2F3030;
 }
 
-#chat-content-viewer > Composite > CopilotTurnWidget > AgentStatusLabel > .text-secondary {
-	color: #A4A4A4;
-}
-
-#chat-content-viewer > Composite > CopilotTurnWidget > AgentToolCancelLabel > Label.text-secondary {
-	color: #A4A4A4;
-}
-
+#chat-content-viewer > Composite > CopilotTurnWidget > AgentStatusLabel > .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > ThinkingBlock .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > .subagent-message-block ThinkingBlock .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > AgentToolCancelLabel > Label.text-secondary,
 #chat-content-viewer > Composite > CopilotTurnWidget > Composite > Label.model-info-label {
 	color: #A4A4A4;
 }

com.microsoft.copilot.eclipse.ui/css/light.css

@@ -115,14 +115,10 @@
 	background-color: #F1F1F2;
 }
 
-#chat-content-viewer > Composite > CopilotTurnWidget > AgentStatusLabel > .text-secondary {
-	color: #808080;
-}
-
-#chat-content-viewer > Composite > CopilotTurnWidget > AgentToolCancelLabel > Label.text-secondary {
-	color: #808080;
-}
-
+#chat-content-viewer > Composite > CopilotTurnWidget > AgentStatusLabel > .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > ThinkingBlock .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > .subagent-message-block ThinkingBlock .text-secondary,
+#chat-content-viewer > Composite > CopilotTurnWidget > AgentToolCancelLabel > Label.text-secondary,
 #chat-content-viewer > Composite > CopilotTurnWidget > Composite > Label.model-info-label {
 	color: #808080;
 }

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/BaseTurnWidget.java

@@ -105,10 +105,16 @@ protected BaseTurnWidget(Composite parent, int style, ChatServiceManager service
     IEventBroker eventBroker = PlatformUI.getWorkbench().getService(IEventBroker.class);
     this.cancelMsgEventHandler = event -> {
       cancelToolConfirmation();
+      onChatMessageCancelled();
     };
     eventBroker.subscribe(CopilotEventConstants.TOPIC_CHAT_MESSAGE_CANCELLED, cancelMsgEventHandler);
   }
 
+  /** Hook invoked when the chat message cancel event is broadcast. Default no-op; subclasses may override. */
+  protected void onChatMessageCancelled() {
+    // no-op
+  }
+
   public String getTurnId() {
     return turnId;
   }
@@ -411,7 +417,8 @@ private void reset() {
     this.currentTextBlock = null;
     this.inCodeBlock = false;
 
-    // Don't reset subagent block state here - it's managed by tool call status
+    // Subagent and thinking blocks have their own lifecycle (tool call status / ThinkingTurnWidget)
+    // and are intentionally not reset here.
   }
 
   /**

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatContentViewer.java

@@ -180,6 +180,14 @@ public void processTurnEvent(ChatProgressValue value) {
       ChatServiceManager chatServiceManager = CopilotUi.getPlugin().getChatServiceManager();
 
       if (value.getKind() == WorkDoneProgressKind.report) {
+        if (turnWidget instanceof ThinkingTurnWidget thinkingTurn) {
+          thinkingTurn.appendThinking(value.getThinking());
+          if (hasRenderableOutput(value)) {
+            // Seal before appending the reply so the spinner stops and the title is fetched.
+            thinkingTurn.sealThinking();
+          }
+        }
+
         if (value.getAgentRounds() != null && !value.getAgentRounds().isEmpty()) {
           // Handle agent mode responses
           AgentRound agentRound = value.getAgentRounds().get(0);
@@ -200,6 +208,10 @@ public void processTurnEvent(ChatProgressValue value) {
           turnWidget.appendMessage(value.getReply());
         }
       } else if (value.getKind() == WorkDoneProgressKind.end) {
+        // Seal any in-progress thinking block before the turn ends.
+        if (turnWidget instanceof ThinkingTurnWidget thinkingTurn) {
+          thinkingTurn.sealThinking();
+        }
         turnWidget.notifyTurnEnd();
       }
       refreshScrollerLayout();
@@ -259,6 +271,29 @@ public void appendMessageToTheLatestTurn(String message) {
     }
   }
 
+  /**
+   * Whether {@code value} carries reply text or an agent round with rendered content; thinking-only reports return
+   * {@code false} so the banner keeps streaming.
+   */
+  private static boolean hasRenderableOutput(ChatProgressValue value) {
+    return StringUtils.isNotBlank(value.getReply()) || hasRenderableAgentRound(value);
+  }
+
+  private static boolean hasRenderableAgentRound(ChatProgressValue value) {
+    if (value.getAgentRounds() == null || value.getAgentRounds().isEmpty()) {
+      return false;
+    }
+    for (AgentRound round : value.getAgentRounds()) {
+      if (StringUtils.isNotBlank(round.getReply())) {
+        return true;
+      }
+      if (round.getToolCalls() != null && !round.getToolCalls().isEmpty()) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   /**
    * Process todo list from tool call result. Extracts todo list data from the tool-specific data
    * and updates the TodoListService.

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatView.java

@@ -823,7 +823,8 @@ public void onChatProgress(ChatProgressValue value) {
           }
         }
         if ((value.getAgentRounds() == null || value.getAgentRounds().isEmpty())
-            && (value.getReply() == null || value.getReply().isEmpty())) {
+            && (value.getReply() == null || value.getReply().isEmpty())
+            && (value.getThinking() == null || StringUtils.isBlank(value.getThinking().text()))) {
           return;
         }
         if (this.chatContentViewer != null) {

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/CopilotTurnWidget.java

@@ -23,12 +23,12 @@
 /**
  * A custom widget that displays a turn for the copilot.
  */
-public class CopilotTurnWidget extends BaseTurnWidget {
+public class CopilotTurnWidget extends ThinkingTurnWidget {
   /**
    * Create the widget.
    */
   public CopilotTurnWidget(Composite parent, int style, ChatServiceManager serviceManager, String turnId) {
-    super(parent, style, serviceManager, turnId, true, null);
+    super(parent, style, serviceManager, turnId, null);
     setData("org.eclipse.swtbot.widget.key", "copilot-turn");
   }
 

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/Messages.java

@@ -37,6 +37,10 @@ public final class Messages extends NLS {
   public static String todoList_clearButtonDisabled;
   public static String todoList_expandTooltip;
   public static String todoList_collapseTooltip;
+  public static String thinking_inProgressTitle;
+  public static String thinking_cancelledTitle;
+  public static String thinking_expandTooltip;
+  public static String thinking_collapseTooltip;
 
   static {
     // initialize resource bundle

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/SubagentMessageBlock.java

@@ -24,7 +24,7 @@ public class SubagentMessageBlock extends Composite {
   private AgentToolCall toolCall;
 
   // Track the current content widget for message processing
-  private BaseTurnWidget currentSubagentTurnWidget;
+  private ThinkingTurnWidget currentSubagentTurnWidget;
 
   /**
    * Create the subagent message block.
@@ -102,7 +102,7 @@ public void notifyTurnEnd() {
    *
    * @return the subagent turn widget
    */
-  public BaseTurnWidget getSubagentTurnWidget() {
+  public ThinkingTurnWidget getSubagentTurnWidget() {
     return currentSubagentTurnWidget;
   }
 }

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/SubagentTurnWidget.java

@@ -19,14 +19,14 @@
  * A turn widget for displaying subagent messages within a SubagentMessageBlock.
  * This widget doesn't show an avatar or role name, only the message content.
  */
-public class SubagentTurnWidget extends BaseTurnWidget {
+public class SubagentTurnWidget extends ThinkingTurnWidget {
 
   /**
    * Create the widget.
    */
   public SubagentTurnWidget(Composite parent, int style, ChatServiceManager serviceManager, String turnId,
       AgentToolCall toolCall) {
-    super(parent, style, serviceManager, turnId + "_subagent", true,
+    super(parent, style, serviceManager, turnId + "_subagent",
         getToolCallRoleName(toolCall));
   }