refactor(desktop): split aux-pane assemblies out of ChannelPane

Rebasing onto current main tripped the repository file-size ratchet:
main's own growth left ChannelPane.tsx at 1011 lines (frozen) and
ChannelScreen.tsx at 1010, so the viewer's pane wiring no longer fits
in-file. Extract the agent-session and markdown-doc pane assemblies into
AgentSessionAuxiliaryPanel and MarkdownDocAuxiliaryPanel (following the
ChannelManagementAuxiliaryPanel pattern), leaving both files under their
caps. No behavior change.

Signed-off-by: Riley Crane <rileycrane@squareup.com>
Co-authored-by: Fizz <d4ec90109b43092b0ec94cf136850915a3732a29ab3089a1dec43d2ed43dd6eb@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

desktop/src/features/channels/ui/AgentSessionAuxiliaryPanel.tsx

@@ -0,0 +1,72 @@
+import { AgentSessionThreadPanel } from "@/features/channels/ui/AgentSessionThreadPanel";
+import * as agentSessionSelection from "@/features/channels/ui/agentSessionSelection";
+import type { ChannelPaneProps } from "@/features/channels/ui/ChannelPane.types";
+
+type AgentSessionAuxiliaryPanelProps = Pick<
+  ChannelPaneProps,
+  "openAgentSessionChannelId" | "profiles"
+> & {
+  activeChannel: NonNullable<ChannelPaneProps["activeChannel"]>;
+  // ChannelPane defaults this prop, so it is always defined at this call site.
+  activityAgents: NonNullable<ChannelPaneProps["activityAgents"]>;
+  isSinglePanelView: boolean;
+  onBack: ChannelPaneProps["onBackFromAgentSession"];
+  onClose: ChannelPaneProps["onCloseAgentSession"];
+  selectedAgent: NonNullable<
+    ReturnType<typeof agentSessionSelection.resolveSelectedAgentSession>
+  >;
+  useSplitAuxiliaryPane: boolean;
+  widthPx: number;
+};
+
+/**
+ * Assembles the agent-session auxiliary pane for ChannelPane's pane chain.
+ * Split out of ChannelPane.tsx to keep it under the per-file line cap.
+ */
+export function AgentSessionAuxiliaryPanel({
+  activeChannel,
+  activityAgents,
+  isSinglePanelView,
+  onBack,
+  onClose,
+  openAgentSessionChannelId,
+  profiles,
+  selectedAgent,
+  useSplitAuxiliaryPane,
+  widthPx,
+}: AgentSessionAuxiliaryPanelProps) {
+  // When the panel was opened from a different channel than the currently
+  // active one, re-scope it to the active channel so that both the
+  // content/header AND channel-backed actions (e.g. Stop current turn)
+  // operate on the same channel object.
+  const effectiveAgentSessionChannelId =
+    openAgentSessionChannelId && activeChannel.id !== openAgentSessionChannelId
+      ? activeChannel.id
+      : openAgentSessionChannelId;
+  return (
+    <AgentSessionThreadPanel
+      agent={selectedAgent}
+      canInterruptTurn={selectedAgent.canInterruptTurn}
+      channel={
+        effectiveAgentSessionChannelId
+          ? effectiveAgentSessionChannelId === activeChannel.id
+            ? activeChannel
+            : null
+          : agentSessionSelection.isAgentInActivityList({
+                activityAgents,
+                selectedAgent,
+              })
+            ? activeChannel
+            : null
+      }
+      channelId={effectiveAgentSessionChannelId}
+      isSinglePanelView={useSplitAuxiliaryPane ? false : isSinglePanelView}
+      layout={useSplitAuxiliaryPane ? "split" : "standalone"}
+      transparentChrome={useSplitAuxiliaryPane}
+      profiles={profiles}
+      onBack={onBack}
+      onClose={onClose}
+      widthPx={widthPx}
+    />
+  );
+}

desktop/src/features/channels/ui/ChannelPane.tsx

@@ -28,9 +28,9 @@ import { buildVideoReviewPresentationByMessageId } from "@/features/messages/lib
 import { isThreadReply } from "@/features/messages/lib/threading";
 import { useComposerHeightPadding } from "@/features/messages/ui/useComposerHeightPadding";
 import { UserProfilePanel } from "@/features/profile/ui/UserProfilePanel";
-import { AgentSessionThreadPanel } from "@/features/channels/ui/AgentSessionThreadPanel";
+import { AgentSessionAuxiliaryPanel } from "@/features/channels/ui/AgentSessionAuxiliaryPanel";
 import { ChannelManagementAuxiliaryPanel } from "@/features/channels/ui/ChannelManagementAuxiliaryPanel";
-import { MarkdownDocPanel } from "@/features/channels/ui/MarkdownDocPanel";
+import { MarkdownDocAuxiliaryPanel } from "@/features/channels/ui/MarkdownDocAuxiliaryPanel";
 import { RightAuxiliaryPane } from "@/features/channels/ui/RightAuxiliaryPane";
 import { ThreadViewModeToggle } from "@/features/channels/ui/ThreadViewModeToggle";
 import { FocusThreadDrawer } from "@/features/channels/ui/FocusThreadDrawer";
@@ -948,42 +948,21 @@ export const ChannelPane = React.memo(function ChannelPane({
             return wrapThreadPanel(panel);
           })()
         ) : activeChannel && selectedAgent ? (
-          (() => {
-            const effectiveAgentSessionChannelId =
-              openAgentSessionChannelId &&
-              activeChannel.id !== openAgentSessionChannelId
-                ? activeChannelId
-                : openAgentSessionChannelId;
-            const panel = (
-              <AgentSessionThreadPanel
-                agent={selectedAgent}
-                canInterruptTurn={selectedAgent.canInterruptTurn}
-                channel={
-                  effectiveAgentSessionChannelId
-                    ? effectiveAgentSessionChannelId === activeChannel.id
-                      ? activeChannel
-                      : null
-                    : agentSessionSelection.isAgentInActivityList({
-                          activityAgents,
-                          selectedAgent,
-                        })
-                      ? activeChannel
-                      : null
-                }
-                channelId={effectiveAgentSessionChannelId}
-                isSinglePanelView={
-                  useSplitAuxiliaryPane ? false : isSinglePanelView
-                }
-                layout={useSplitAuxiliaryPane ? "split" : "standalone"}
-                transparentChrome={useSplitAuxiliaryPane}
-                profiles={profiles}
-                onBack={onBackFromAgentSession}
-                onClose={onCloseAgentSession}
-                widthPx={threadPanelWidthPx}
-              />
-            );
-            return wrapAux(panel, "agent-session-thread-panel");
-          })()
+          wrapAux(
+            <AgentSessionAuxiliaryPanel
+              activeChannel={activeChannel}
+              activityAgents={activityAgents}
+              isSinglePanelView={isSinglePanelView}
+              onBack={onBackFromAgentSession}
+              onClose={onCloseAgentSession}
+              openAgentSessionChannelId={openAgentSessionChannelId}
+              profiles={profiles}
+              selectedAgent={selectedAgent}
+              useSplitAuxiliaryPane={useSplitAuxiliaryPane}
+              widthPx={threadPanelWidthPx}
+            />,
+            "agent-session-thread-panel",
+          )
         ) : profilePanelPubkey ? (
           (() => {
             const panel = (
@@ -1009,29 +988,16 @@ export const ChannelPane = React.memo(function ChannelPane({
             return wrapAux(panel, "user-profile-panel");
           })()
         ) : openMarkdownDoc && onCloseMarkdownDoc ? (
-          (() => {
-            // Lowest-priority pane: a higher-priority pane opened afterwards
-            // (thread, activity, profile) shows immediately, and the document
-            // reappears when it closes. Opening a document clears competitors
-            // in the screen-level handler, so it is never dead on arrival.
-            const panel = (
-              // Keyed by URL so opening a different document resets the
-              // Preview/Code toggle instead of inheriting the previous doc's.
-              <MarkdownDocPanel
-                key={openMarkdownDoc.url}
-                filename={openMarkdownDoc.filename}
-                isSinglePanelView={
-                  useSplitAuxiliaryPane ? false : isSinglePanelView
-                }
-                layout={useSplitAuxiliaryPane ? "split" : "standalone"}
-                onClose={onCloseMarkdownDoc}
-                transparentChrome={useSplitAuxiliaryPane}
-                url={openMarkdownDoc.url}
-                widthPx={threadPanelWidthPx}
-              />
-            );
-            return wrapAux(panel, "markdown-doc-panel");
-          })()
+          wrapAux(
+            <MarkdownDocAuxiliaryPanel
+              doc={openMarkdownDoc}
+              isSinglePanelView={isSinglePanelView}
+              onClose={onCloseMarkdownDoc}
+              useSplitAuxiliaryPane={useSplitAuxiliaryPane}
+              widthPx={threadPanelWidthPx}
+            />,
+            "markdown-doc-panel",
+          )
         ) : null}
       </AnimatePresence>
     </div>

desktop/src/features/channels/ui/ChannelScreen.tsx

@@ -833,9 +833,7 @@ export function ChannelScreen({
                 searchTarget,
               )
             ) : (
-              // Scoped to the pane that hosts the markdown-doc auxiliary
-              // panel — the forum branch has no host, so its FileCards keep
-              // download behavior instead of a dead open-in-viewer click.
+              // Forum has no doc-panel host; its FileCards keep downloading.
               <MarkdownDocViewerProvider value={handleOpenMarkdownDoc}>
                 <React.Suspense
                   fallback={

desktop/src/features/channels/ui/MarkdownDocAuxiliaryPanel.tsx

@@ -0,0 +1,43 @@
+import { MarkdownDocPanel } from "@/features/channels/ui/MarkdownDocPanel";
+import type { MarkdownDocTarget } from "@/shared/ui/markdown/markdownDocViewerContext";
+
+type MarkdownDocAuxiliaryPanelProps = {
+  doc: MarkdownDocTarget;
+  isSinglePanelView: boolean;
+  onClose: () => void;
+  useSplitAuxiliaryPane: boolean;
+  widthPx: number;
+};
+
+/**
+ * Assembles the markdown-document auxiliary pane for ChannelPane's pane
+ * chain. Split out of ChannelPane.tsx to keep it under the per-file line
+ * cap.
+ *
+ * This is the chain's lowest-priority pane: a higher-priority pane opened
+ * afterwards (thread, activity, profile) shows immediately, and the document
+ * reappears when it closes. Opening a document clears competitors in the
+ * screen-level handler, so it is never dead on arrival.
+ */
+export function MarkdownDocAuxiliaryPanel({
+  doc,
+  isSinglePanelView,
+  onClose,
+  useSplitAuxiliaryPane,
+  widthPx,
+}: MarkdownDocAuxiliaryPanelProps) {
+  return (
+    // Keyed by URL so opening a different document resets the Preview/Code
+    // toggle instead of inheriting the previous document's.
+    <MarkdownDocPanel
+      key={doc.url}
+      filename={doc.filename}
+      isSinglePanelView={useSplitAuxiliaryPane ? false : isSinglePanelView}
+      layout={useSplitAuxiliaryPane ? "split" : "standalone"}
+      onClose={onClose}
+      transparentChrome={useSplitAuxiliaryPane}
+      url={doc.url}
+      widthPx={widthPx}
+    />
+  );
+}