refactor(ui): dedupe git file helpers and memoize git view model

- N1: extract the copy-pasted pointerName/segmentName helpers from
  gitFileList.ts and gitFileTree.ts into gitFileShared.ts.
- N3: drop the dead conditional "tree" class (no CSS rule exists).
- N4 (P3): memoize computeViewState on (status, view) identity so renders
  from expand/collapse or diff selection skip the full model rebuild;
  expand state is read live at render time, never cached.

src/client/src/components/WorkspaceGitPanel.ts

@@ -13,6 +13,12 @@ interface GitViewState {
   readonly expandablePaths: readonly string[];
 }
 
+interface GitViewStateCache {
+  readonly status: GitStatusResponse | undefined;
+  readonly view: GitFileView;
+  readonly viewState: GitViewState;
+}
+
 const EMPTY_LIST_MODEL: GitFileListModel = { submodules: [], files: [] };
 const EMPTY_VIEW_STATE: GitViewState = { nodes: [], listModel: EMPTY_LIST_MODEL, expandablePaths: [] };
 
@@ -45,6 +51,11 @@ export class WorkspaceGitPanel extends LitElement {
   // submodule groups, both keyed by their path.
   @state() private expandedDirectories = new Set<string>();
 
+  // Memoized on (status, view) identity: renders triggered by expand/collapse
+  // or diff selection reuse the model, while a new poll or view switch rebuilds
+  // it. Expand state is read live at render time, never baked into the model.
+  private viewStateCache: GitViewStateCache | undefined;
+
   protected override willUpdate(changedProperties: PropertyValues<this>): void {
     if (!changedProperties.has("context")) return;
     const previous = changedProperties.get("context");
@@ -70,7 +81,7 @@ export class WorkspaceGitPanel extends LitElement {
         </div>
       </section>
       <section class="split">
-        <div class=${this.view === "tree" ? "list tree" : "list"}>
+        <div class="list">
           ${this.renderFileList(context, status, viewState)}
         </div>
         <div class="viewer">${renderDiffViewer(context)}</div>
@@ -166,6 +177,14 @@ export class WorkspaceGitPanel extends LitElement {
   }
 
   private computeViewState(status: GitStatusResponse | undefined): GitViewState {
+    const cached = this.viewStateCache;
+    if (cached !== undefined && cached.status === status && cached.view === this.view) return cached.viewState;
+    const viewState = this.buildViewState(status);
+    this.viewStateCache = { status, view: this.view, viewState };
+    return viewState;
+  }
+
+  private buildViewState(status: GitStatusResponse | undefined): GitViewState {
     if (status === undefined || !status.isGitRepo || status.files.length === 0) return EMPTY_VIEW_STATE;
     if (this.view === "tree") {
       const nodes = buildGitFileTree(status.files, status.submodules);

src/client/src/gitFileList.ts

@@ -1,4 +1,5 @@
 import type { GitStatusFile } from "./api";
+import { pointerName, segmentName } from "./gitFileShared";
 
 /** A changed file inside a submodule, shown flat in list view. `path` is the
  * full superproject-relative path (the diff key); `relativePath` is shown in
@@ -70,15 +71,3 @@ function ownerSubmodule(path: string, submodules: readonly string[]): string | u
   }
   return best;
 }
-
-function pointerName(file: GitStatusFile): string {
-  const from = file.submoduleFromCommit;
-  const to = file.submoduleToCommit;
-  return from !== undefined && to !== undefined ? `${from} → ${to}` : "commit";
-}
-
-function segmentName(path: string): string {
-  const segments = path.split("/");
-  const last = segments[segments.length - 1];
-  return last ?? path;
-}

src/client/src/gitFileShared.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from "vitest";
+import type { GitStatusFile } from "./api";
+import { pointerName, segmentName } from "./gitFileShared";
+
+describe("pointerName", () => {
+  it("renders the moved pointer as <from> → <to>", () => {
+    expect(pointerName(pointer("1111111", "2222222"))).toBe("1111111 → 2222222");
+  });
+
+  it("falls back to a plain label when either end is unresolved", () => {
+    expect(pointerName(pointer(undefined, "2222222"))).toBe("commit");
+    expect(pointerName(pointer("1111111", undefined))).toBe("commit");
+    expect(pointerName(pointer(undefined, undefined))).toBe("commit");
+  });
+});
+
+describe("segmentName", () => {
+  it("returns the final path segment", () => {
+    expect(segmentName("deps/sub/module")).toBe("module");
+  });
+
+  it("returns the whole path when there is no separator", () => {
+    expect(segmentName("HARL")).toBe("HARL");
+  });
+});
+
+function pointer(from: string | undefined, to: string | undefined): GitStatusFile {
+  return {
+    path: "HARL",
+    index: "unmodified",
+    workingTree: "modified",
+    ...(from === undefined ? {} : { submoduleFromCommit: from }),
+    ...(to === undefined ? {} : { submoduleToCommit: to }),
+  };
+}

src/client/src/gitFileShared.ts

@@ -0,0 +1,18 @@
+import type { GitStatusFile } from "./api";
+
+/**
+ * Display label for a submodule commit-pointer row: the `<old> → <new>`
+ * short-SHA summary, or "commit" when the server did not resolve both ends.
+ */
+export function pointerName(file: GitStatusFile): string {
+  const from = file.submoduleFromCommit;
+  const to = file.submoduleToCommit;
+  return from !== undefined && to !== undefined ? `${from} → ${to}` : "commit";
+}
+
+/** Final segment of a path, used as the display name for tree/list rows. */
+export function segmentName(path: string): string {
+  const segments = path.split("/");
+  const last = segments[segments.length - 1];
+  return last ?? path;
+}

src/client/src/gitFileTree.ts

@@ -1,4 +1,5 @@
 import type { GitStatusFile } from "./api";
+import { pointerName, segmentName } from "./gitFileShared";
 
 /**
  * A changed file placed at a leaf of the Git file tree. `path` is the full
@@ -93,12 +94,6 @@ export function collectGitFileTreeDirectoryPaths(nodes: readonly GitFileTreeNode
   return paths;
 }
 
-function pointerName(file: GitStatusFile): string {
-  const from = file.submoduleFromCommit;
-  const to = file.submoduleToCommit;
-  return from !== undefined && to !== undefined ? `${from} → ${to}` : "commit";
-}
-
 function createDirectoryAccumulator(path: string): DirectoryAccumulator {
   return { path, directories: new Map(), files: [], isSubmodule: false };
 }
@@ -125,9 +120,3 @@ function finalizeChildren(directory: DirectoryAccumulator): GitFileTreeNode[] {
   const files = [...directory.files].sort((left, right) => left.name.localeCompare(right.name));
   return [...(directory.pointer === undefined ? [] : [directory.pointer]), ...directories, ...files];
 }
-
-function segmentName(path: string): string {
-  const segments = path.split("/");
-  const last = segments[segments.length - 1];
-  return last ?? path;
-}