From ef0ce15bb53a9bdfdfecf090324e317d8c2f9b33 Mon Sep 17 00:00:00 2001
From: syntaxbullet <syntaxbullet@protonmail.com>
Date: Sat, 4 Jul 2026 15:37:54 +0200
Subject: [PATCH] perf(view): reduce shell rerenders

---
 view/App.tsx                              |  76 +++++++++------
 view/CanvasViewport.tsx                   |  57 ++++++++++-
 view/GenerateSheet.tsx                    |  17 ++--
 view/LayersSheet.tsx                      |  43 +++++++--
 view/bottom-controls/GenerateControls.tsx | 112 ----------------------
 view/canvas/brush.ts                      |  18 ++--
 view/generate/runGenerate.ts              |   2 +-
 view/useAppState.test.ts                  |  68 +++++++++++++
 view/useAppState.ts                       | 101 ++++++++++++++++++-
 9 files changed, 321 insertions(+), 173 deletions(-)
 create mode 100644 view/useAppState.test.ts

diff --git a/view/App.tsx b/view/App.tsx
index 2405d9d..d863302 100644
--- a/view/App.tsx
+++ b/view/App.tsx
@@ -10,9 +10,10 @@ import { LayersSheet } from "./LayersSheet";
 import { ShortcutsDisplay } from "./ShortcutsDisplay";
 import { ToolOverlay } from "./ToolOverlay";
 import { resolveTransformTargetBounds, selectedTransformTarget } from "@editor/transform-targets";
+import type { AppState } from "@editor/state";
 import type { ToolId } from "@editor/tools";
 import { handleDeleteSelectionKey, handleHistoryKey, handleToolKey, keybindEventFromKeyboardEvent } from "@input/index";
-import { useAppState } from "./useAppState";
+import { shallowEqual, useAppState } from "./useAppState";
 import { downloadArtboardPng } from "./exportArtboardPng";
 import { useImageImport } from "./useImageImport";
 import { useViewportActivityIsland } from "./useViewportActivityIsland";
@@ -23,14 +24,15 @@ export type AppProps = {
 };
 
 export function App({ app }: AppProps) {
-  const state = useAppState(app.store);
-  const viewportActivityIsland = useViewportActivityIsland(state.editor.viewport);
+  const shellState = useAppState(app.store, selectAppShellState, shallowEqual);
+  const { document, selection, viewport, tools, transformSession, maskEdit } = shellState;
+  const viewportActivityIsland = useViewportActivityIsland(viewport);
   const imageImport = useImageImport(app.store);
   const [layersOpen, setLayersOpen] = useState(false);
   const [generateOpen, setGenerateOpen] = useState(false);
   const previousGenerateTool = useRef<ToolId>("select");
-  const transformTarget = state.editor.transformSession?.target ?? selectedTransformTarget(state.document, state.editor.selection);
-  const activeArtboard = state.document.artboards.find((artboard) => artboard.id === state.editor.selection.artboardId) ?? state.document.artboards[0];
+  const transformTarget = transformSession?.target ?? selectedTransformTarget(document, selection);
+  const activeArtboard = document.artboards.find((artboard) => artboard.id === selection.artboardId) ?? document.artboards[0];
 
   const openGenerate = useCallback(() => {
     const activeTool = app.store.getState().editor.tools.activeTool;
@@ -108,8 +110,8 @@ export function App({ app }: AppProps) {
     window.addEventListener("keydown", handleKeyDown);
     return () => window.removeEventListener("keydown", handleKeyDown);
   }, [app.store, closeGenerate, toggleGenerate]);
-  const transformBounds = transformTarget ? resolveTransformTargetBounds(state.document, transformTarget) : undefined;
-  const brushHint = brushUnavailableHint(state.document, state.editor);
+  const transformBounds = transformTarget ? resolveTransformTargetBounds(document, transformTarget) : undefined;
+  const brushHint = brushUnavailableHint(document, { selection, tools, maskEdit });
 
   return (
     <main className="relative h-full overflow-hidden bg-[radial-gradient(circle_at_20%_18%,rgba(148,163,184,0.18),transparent_34%),radial-gradient(circle_at_82%_22%,rgba(71,85,105,0.22),transparent_36%),radial-gradient(circle_at_48%_88%,rgba(30,41,59,0.28),transparent_40%),linear-gradient(135deg,#020617_0%,#0f172a_46%,#111827_100%)] text-foreground">
@@ -122,7 +124,7 @@ export function App({ app }: AppProps) {
           <button type="button" className={topBarButtonClass(generateOpen)} aria-pressed={generateOpen} onClick={toggleGenerate}>
             <Sparkle size={24} weight={generateOpen ? "fill" : "regular"} />
           </button>
-          <button type="button" className={topBarButtonClass()} disabled={!activeArtboard} onClick={() => activeArtboard && void downloadArtboardPng(activeArtboard, state.document.assets)}>
+          <button type="button" className={topBarButtonClass()} disabled={!activeArtboard} onClick={() => activeArtboard && void downloadArtboardPng(activeArtboard, document.assets)}>
             <DownloadSimple size={24} />
           </button>
           <button type="button" className={topBarButtonClass(layersOpen)} aria-pressed={layersOpen} onClick={() => { closeGenerate(); setLayersOpen((open) => !open); }}>
@@ -132,41 +134,37 @@ export function App({ app }: AppProps) {
       </header>
       <div className="absolute left-3 top-1/2 z-10 -translate-y-1/2">
         <ToolOverlay
-          activeTool={state.editor.tools.activeTool}
-          interactionMode={state.editor.tools.interactionMode}
+          activeTool={tools.activeTool}
+          interactionMode={tools.interactionMode}
           dispatch={app.store.dispatch}
         />
       </div>
       <GenerateSheet
-        document={state.document}
-        selection={state.editor.selection}
-        viewport={state.editor.viewport}
-        settings={state.editor.tools.generate}
+        settings={tools.generate}
         open={generateOpen}
-        onOpenChange={(open) => open ? openGenerate() : closeGenerate()}
         dispatch={app.store.dispatch}
       />
       <LayersSheet
-        document={state.document}
-        selection={state.editor.selection}
-        maskEdit={state.editor.maskEdit}
+        document={document}
+        selection={selection}
+        maskEdit={maskEdit}
         open={layersOpen}
         dispatch={app.store.dispatch}
       />
       <div className="absolute inset-x-0 bottom-4 z-10 flex justify-center">
         <BottomControlsIsland
-          document={state.document}
-          selection={state.editor.selection}
-          viewport={state.editor.viewport}
-          visible={state.editor.tools.activeTool === "generate" || state.editor.tools.activeTool === "brush" || state.editor.tools.activeTool === "eraser" || state.editor.tools.activeTool === "chromaKey" || state.editor.tools.activeTool === "magicWand" || Boolean(transformBounds) || viewportActivityIsland.visible}
+          document={document}
+          selection={selection}
+          viewport={viewport}
+          visible={tools.activeTool === "generate" || tools.activeTool === "brush" || tools.activeTool === "eraser" || tools.activeTool === "chromaKey" || tools.activeTool === "magicWand" || Boolean(transformBounds) || viewportActivityIsland.visible}
           action={viewportActivityIsland.action}
-          activeTool={state.editor.tools.activeTool}
-          brushSettings={state.editor.tools.brush}
-          generateSettings={state.editor.tools.generate}
-          chromaKeySettings={state.editor.tools.chromaKey}
-          magicWandSettings={state.editor.tools.magicWand}
-          editingMask={Boolean(state.editor.maskEdit)}
-          maskViewMode={state.editor.maskEdit?.viewMode ?? "composite"}
+          activeTool={tools.activeTool}
+          brushSettings={tools.brush}
+          generateSettings={tools.generate}
+          chromaKeySettings={tools.chromaKey}
+          magicWandSettings={tools.magicWand}
+          editingMask={Boolean(maskEdit)}
+          maskViewMode={maskEdit?.viewMode ?? "composite"}
           brushHint={brushHint}
           transformBounds={viewportActivityIsland.visible ? undefined : transformBounds}
           transformTarget={viewportActivityIsland.visible ? undefined : transformTarget}
@@ -186,4 +184,24 @@ function topBarButtonClass(active = false) {
   return active ? `${base} bg-white text-black hover:bg-white hover:text-black` : `${base} text-white/75 hover:bg-white/10 hover:text-white`;
 }
 
+type AppShellState = {
+  document: AppState["document"];
+  selection: AppState["editor"]["selection"];
+  viewport: AppState["editor"]["viewport"];
+  tools: AppState["editor"]["tools"];
+  transformSession: AppState["editor"]["transformSession"];
+  maskEdit: AppState["editor"]["maskEdit"];
+};
+
+function selectAppShellState(state: AppState): AppShellState {
+  return {
+    document: state.document,
+    selection: state.editor.selection,
+    viewport: state.editor.viewport,
+    tools: state.editor.tools,
+    transformSession: state.editor.transformSession,
+    maskEdit: state.editor.maskEdit,
+  };
+}
+
 export default App;
diff --git a/view/CanvasViewport.tsx b/view/CanvasViewport.tsx
index 318e9e2..980c59a 100644
--- a/view/CanvasViewport.tsx
+++ b/view/CanvasViewport.tsx
@@ -1,7 +1,10 @@
 import { useMemo, useRef } from "react";
+import type { ImageDocument } from "@core/document";
+import type { AppState, MaskEditState } from "@editor/state";
 import type { AppStore } from "@editor/store";
+import type { InteractionMode } from "@editor/tools";
 import type { GlobalKeybindConsumer, GlobalPointerConsumer, GlobalWheelConsumer } from "@input/index";
-import { brushUnavailableHint, canPreviewBrush } from "./canvas/brush";
+import { brushUnavailableHint, canPreviewBrush, type BrushTargetEditorState } from "./canvas/brush";
 import { canvasCursorClass } from "./canvas/cursor";
 import { useCanvasInput } from "./canvas/useCanvasInput";
 import { useCanvasRenderer } from "./canvas/useCanvasRenderer";
@@ -26,7 +29,7 @@ export function CanvasViewport({
   globalWheelConsumer = ignoreGlobalWheel,
 }: CanvasViewportProps) {
   const canvasRef = useRef<HTMLCanvasElement | null>(null);
-  const state = useAppState(store);
+  const cursorState = useAppState(store, selectCanvasCursorState, canvasCursorStatesEqual);
   const inputOptions = useMemo(
     () => ({ globalKeybindConsumer, globalPointerConsumer, globalWheelConsumer }),
     [globalKeybindConsumer, globalPointerConsumer, globalWheelConsumer],
@@ -35,9 +38,53 @@ export function CanvasViewport({
   useCanvasRenderer(canvasRef, store);
   useCanvasResize(canvasRef, store.dispatch);
   const input = useCanvasInput(canvasRef, store, inputOptions);
-  const brushHint = brushUnavailableHint(state.document, state.editor);
-  const hasBrushPreview = Boolean(state.editor.brushPreview && !brushHint && canPreviewBrush(state.document, state.editor));
-  const cursorClass = canvasCursorClass(state.editor.tools.interactionMode, input, hasBrushPreview, !brushHint);
+  const brushHint = brushUnavailableHint(cursorState.document, cursorState.editor);
+  const hasBrushPreview = Boolean(cursorState.hasBrushPreview && !brushHint && canPreviewBrush(cursorState.document, cursorState.editor));
+  const cursorClass = canvasCursorClass(cursorState.editor.tools.interactionMode, input, hasBrushPreview, !brushHint);
 
   return <canvas ref={canvasRef} className={`h-full w-full ${cursorClass}`} />;
 }
+
+type CanvasCursorState = {
+  document: ImageDocument;
+  editor: BrushTargetEditorState;
+  hasBrushPreview: boolean;
+};
+
+function selectCanvasCursorState(state: AppState): CanvasCursorState {
+  return {
+    document: state.document,
+    editor: {
+      selection: state.editor.selection,
+      tools: {
+        activeTool: state.editor.tools.activeTool,
+        interactionMode: state.editor.tools.interactionMode,
+      },
+      maskEdit: state.editor.maskEdit,
+    },
+    hasBrushPreview: Boolean(state.editor.brushPreview),
+  };
+}
+
+function canvasCursorStatesEqual(a: CanvasCursorState, b: CanvasCursorState): boolean {
+  return (
+    a.document === b.document &&
+    a.hasBrushPreview === b.hasBrushPreview &&
+    a.editor.selection === b.editor.selection &&
+    a.editor.tools.activeTool === b.editor.tools.activeTool &&
+    interactionModesEqual(a.editor.tools.interactionMode, b.editor.tools.interactionMode) &&
+    maskEditStatesEqual(a.editor.maskEdit, b.editor.maskEdit)
+  );
+}
+
+function interactionModesEqual(a: InteractionMode, b: InteractionMode): boolean {
+  if (a.type !== b.type) return false;
+  if (a.type === "temporary-pan") return b.type === "temporary-pan" && a.previousTool === b.previousTool;
+  return b.type === "tool" && a.tool === b.tool;
+}
+
+function maskEditStatesEqual(a: MaskEditState | undefined, b: MaskEditState | undefined): boolean {
+  if (a === b) return true;
+  if (!a || !b) return false;
+  return a.targetLayerId === b.targetLayerId && a.maskLayerId === b.maskLayerId && a.viewMode === b.viewMode;
+}
diff --git a/view/GenerateSheet.tsx b/view/GenerateSheet.tsx
index aef19d1..5bb9f64 100644
--- a/view/GenerateSheet.tsx
+++ b/view/GenerateSheet.tsx
@@ -1,20 +1,14 @@
-import type { ImageDocument } from "@core/document";
-import type { SelectionState, ViewportState } from "@editor/state";
 import type { GenerateSettings } from "@editor/tools";
 import type { AppStore } from "@editor/store";
 import { GenerateControls } from "./bottom-controls/GenerateControls";
 
 export type GenerateSheetProps = {
-  document: ImageDocument;
-  selection: SelectionState;
-  viewport: ViewportState;
   settings: GenerateSettings;
   open: boolean;
-  onOpenChange: (open: boolean) => void;
   dispatch: AppStore["dispatch"];
 };
 
-export function GenerateSheet({ document, selection, viewport, settings, open, onOpenChange, dispatch }: GenerateSheetProps) {
+export function GenerateSheet({ settings, open, dispatch }: GenerateSheetProps) {
   return (
     <aside
       aria-hidden={!open}
@@ -23,10 +17,11 @@ export function GenerateSheet({ document, selection, viewport, settings, open, o
         open ? "translate-x-0 opacity-100" : "pointer-events-none translate-x-8 opacity-0"
       }`}
     >
-      <div className="subtle-scrollbar min-h-0 flex-1 overflow-auto py-4">
-        <GenerateControls document={document} selection={selection} viewport={viewport} settings={settings} dispatch={dispatch} />
-      </div>
+      {open ? (
+        <div className="subtle-scrollbar min-h-0 flex-1 overflow-auto py-4">
+          <GenerateControls settings={settings} dispatch={dispatch} />
+        </div>
+      ) : null}
     </aside>
   );
 }
-
diff --git a/view/LayersSheet.tsx b/view/LayersSheet.tsx
index 3916ae5..8a2ef38 100644
--- a/view/LayersSheet.tsx
+++ b/view/LayersSheet.tsx
@@ -19,11 +19,6 @@ export type LayersSheetProps = {
 };
 
 export function LayersSheet({ document, selection, maskEdit, open, dispatch }: LayersSheetProps) {
-  const selectedArtboardId = selection.artboardId ?? document.artboards[0]?.id;
-  const selectedLayer = findLayerInfoInDocument(document, selection.layerIds[0]);
-  const canGroup = Boolean(selection.artboardId && selection.layerIds.length > 0);
-  const canUngroup = selectedLayer?.layer.type === "group";
-  const maskLayerIds = collectDocumentMaskLayerIds(document);
   const draggedLayerId = useRef<string>();
   const [editingTitle, setEditingTitle] = useState<EditingTitle>();
 
@@ -34,6 +29,42 @@ export function LayersSheet({ document, selection, maskEdit, open, dispatch }: L
         open ? "translate-x-0 opacity-100" : "pointer-events-none translate-x-8 opacity-0"
       }`}
     >
+      {open ? (
+        <LayersSheetBody
+          document={document}
+          selection={selection}
+          maskEdit={maskEdit}
+          draggedLayerId={draggedLayerId}
+          editingTitle={editingTitle}
+          setEditingTitle={setEditingTitle}
+          dispatch={dispatch}
+        />
+      ) : null}
+    </aside>
+  );
+}
+
+function LayersSheetBody({
+  document,
+  selection,
+  maskEdit,
+  draggedLayerId,
+  editingTitle,
+  setEditingTitle,
+  dispatch,
+}: Omit<LayersSheetProps, "open"> & {
+  draggedLayerId: MutableRefObject<string | undefined>;
+  editingTitle: EditingTitle | undefined;
+  setEditingTitle: (editingTitle: EditingTitle | undefined) => void;
+}) {
+  const selectedArtboardId = selection.artboardId ?? document.artboards[0]?.id;
+  const selectedLayer = findLayerInfoInDocument(document, selection.layerIds[0]);
+  const canGroup = Boolean(selection.artboardId && selection.layerIds.length > 0);
+  const canUngroup = selectedLayer?.layer.type === "group";
+  const maskLayerIds = collectDocumentMaskLayerIds(document);
+
+  return (
+    <>
       <header className="flex h-20 items-center">
         <div className="flex w-full items-center gap-2">
           <button type="button" className={labeledToolbarButtonClass()} aria-label="Add artboard" title="Add artboard" onClick={() => addArtboard(document, dispatch)}>
@@ -145,7 +176,7 @@ export function LayersSheet({ document, selection, maskEdit, open, dispatch }: L
           </section>
         ))}
       </div>
-    </aside>
+    </>
   );
 }
 
diff --git a/view/bottom-controls/GenerateControls.tsx b/view/bottom-controls/GenerateControls.tsx
index c2b2ac4..c3a1014 100644
--- a/view/bottom-controls/GenerateControls.tsx
+++ b/view/bottom-controls/GenerateControls.tsx
@@ -1,10 +1,7 @@
 import { useEffect, useRef, useState, type RefObject } from "react";
 import { CaretDown, CaretUp } from "@phosphor-icons/react";
 import { commandIds } from "@commands/ids";
-import type { ImageDocument } from "@core/document";
-import type { Layer } from "@core/layer";
 import type { AppStore } from "@editor/store";
-import type { SelectionState, ViewportState } from "@editor/state";
 import type { GenerateMode, GenerateModel, GenerateSettings } from "@editor/tools";
 import { BottomControlSelectMenu, type BottomControlSelectOption } from "./SelectMenu";
 import { BottomControlSlider } from "./Slider";
@@ -25,9 +22,6 @@ const sizePresets = [
 ] as const;
 
 export type GenerateControlsProps = {
-  document: ImageDocument;
-  selection: SelectionState;
-  viewport: ViewportState;
   settings: GenerateSettings;
   dispatch: AppStore["dispatch"];
 };
@@ -226,109 +220,3 @@ function panelLabelClass() {
 function panelTextAreaClass(extra = "") {
   return `${extra} resize-none rounded-[1.25rem] bg-white/5 px-4 py-3 text-sm text-white outline-none transition placeholder:text-white/25 focus:bg-white/[0.07] focus:ring-2 focus:ring-white/30`;
 }
-
-async function generateImage(options: GenerateControlsProps & { setBusy: (busy: boolean) => void; setError: (error: string | undefined) => void }) {
-  const { document, selection, viewport, settings, dispatch, setBusy, setError } = options;
-  const artboard = selection.artboardId ? document.artboards.find((candidate) => candidate.id === selection.artboardId) : document.artboards[0];
-  if (!artboard) return;
-
-  setBusy(true);
-  setError(undefined);
-  try {
-    const target = resolveSelectedImage(document, selection);
-    const inputImage = target && settings.mode !== "text-to-image" ? await imageSourceToDataUrl(target.asset.source) : undefined;
-    const maskImage = target?.maskAsset && settings.mode === "inpaint" ? await imageSourceToDataUrl(target.maskAsset.source) : undefined;
-    const response = await fetch("/api/comfy/generate", {
-      method: "POST",
-      headers: { "content-type": "application/json" },
-      body: JSON.stringify({
-        mode: settings.mode,
-        model: settings.model,
-        prompt: settings.prompt,
-        negativePrompt: settings.negativePrompt,
-        strength: settings.strength,
-        steps: settings.steps,
-        cfg: settings.cfg,
-        seed: settings.seed,
-        sampler: settings.sampler,
-        scheduler: settings.scheduler,
-        width: settings.width,
-        height: settings.height,
-        outpaint: settings.outpaint,
-        inputImage,
-        maskImage,
-      }),
-    });
-    if (!response.ok) throw new Error(await response.text());
-    const generated = await response.json() as { source: string; mimeType: string };
-    const intrinsicSize = await loadImageSize(generated.source);
-    const assetId = crypto.randomUUID();
-    const layerId = crypto.randomUUID();
-    dispatch(commandIds.documentAddAsset, { asset: { id: assetId, name: "Generated image", mimeType: generated.mimeType, source: generated.source, intrinsicSize } });
-    dispatch(commandIds.documentAddImageLayer, {
-      artboardId: artboard.id,
-      layer: {
-        id: layerId,
-        type: "image",
-        name: "Generated image",
-        visible: true,
-        locked: false,
-        opacity: 1,
-        assetId,
-        transform: { position: { x: viewport.center.x - intrinsicSize.w / 2, y: viewport.center.y - intrinsicSize.h / 2 }, scale: { x: 1, y: 1 }, rotation: 0 },
-      },
-    });
-    dispatch(commandIds.selectionSet, { artboardId: artboard.id, layerIds: [layerId] });
-  } catch (reason) {
-    setError(reason instanceof Error ? reason.message : "Generation failed");
-  } finally {
-    setBusy(false);
-  }
-}
-
-function resolveSelectedImage(document: ImageDocument, selection: SelectionState) {
-  const layerId = selection.layerIds[0];
-  if (!layerId) return undefined;
-  const layer = findLayer(document.artboards.find((artboard) => artboard.id === selection.artboardId)?.layers ?? [], layerId);
-  if (!layer || layer.type === "group") return undefined;
-  const asset = document.assets.find((candidate) => candidate.id === layer.assetId);
-  const maskLayer = layer.clippingMask ? findLayer(document.artboards.flatMap((artboard) => artboard.layers), layer.clippingMask.maskLayerId) : undefined;
-  const maskAsset = maskLayer && maskLayer.type !== "group" ? document.assets.find((candidate) => candidate.id === maskLayer.assetId) : undefined;
-  return asset ? { layer, asset, maskAsset } : undefined;
-}
-
-function findLayer(layers: readonly Layer[], layerId: string): Layer | undefined {
-  for (const layer of layers) {
-    if (layer.id === layerId) return layer;
-    if (layer.type === "group") {
-      const found = findLayer(layer.children, layerId);
-      if (found) return found;
-    }
-  }
-  return undefined;
-}
-
-async function imageSourceToDataUrl(source: string) {
-  if (source.startsWith("data:")) return source;
-  const image = await loadImage(source);
-  const canvas = document.createElement("canvas");
-  canvas.width = image.naturalWidth;
-  canvas.height = image.naturalHeight;
-  const context = canvas.getContext("2d");
-  if (!context) throw new Error("Unable to read selected image");
-  context.drawImage(image, 0, 0);
-  return canvas.toDataURL("image/png");
-}
-
-function loadImageSize(source: string): Promise<{ w: number; h: number }> {
-  return loadImage(source).then((image) => ({ w: image.naturalWidth, h: image.naturalHeight }));
-}
-
-function loadImage(source: string): Promise<HTMLImageElement> {
-  return new Promise((resolve, reject) => {
-    const image = new Image();
-    image.onload = () => resolve(image);
-    image.onerror = () => reject(new Error("Failed to load image"));
-    image.src = source;
-  });
-}
diff --git a/view/canvas/brush.ts b/view/canvas/brush.ts
index cec7161..bc98cfa 100644
--- a/view/canvas/brush.ts
+++ b/view/canvas/brush.ts
@@ -3,8 +3,8 @@ import type { ImageDocument } from "@core/document";
 import type { Vec2D } from "@core/geometry";
 import type { Layer } from "@core/layer";
 import type { RasterLayer } from "@core/raster-layer";
-import type { EditorState } from "@editor/state";
-import { isPanInteractionMode } from "@editor/tools";
+import type { MaskEditState, SelectionState } from "@editor/state";
+import { isPanInteractionMode, type ToolState } from "@editor/tools";
 import type { AppStore } from "@editor/store";
 
 export type BrushSession = {
@@ -17,17 +17,23 @@ export type BrushSession = {
   cancelled?: boolean;
 };
 
-export function beginBrushSession(document: ImageDocument, editor: EditorState, point: Vec2D): BrushSession | undefined {
+export type BrushTargetEditorState = {
+  selection: SelectionState;
+  tools: Pick<ToolState, "activeTool" | "interactionMode">;
+  maskEdit?: MaskEditState;
+};
+
+export function beginBrushSession(document: ImageDocument, editor: BrushTargetEditorState, point: Vec2D): BrushSession | undefined {
   const layer = resolveBrushTargetLayer(document, editor);
   if (!layer || (editor.tools.activeTool !== "brush" && editor.tools.activeTool !== "eraser")) return undefined;
   return { layerId: layer.id, assetId: layer.assetId, previousPoint: point, mode: editor.tools.activeTool };
 }
 
-export function canPreviewBrush(document: ImageDocument, editor: EditorState): boolean {
+export function canPreviewBrush(document: ImageDocument, editor: BrushTargetEditorState): boolean {
   return Boolean(resolveBrushTargetLayer(document, editor));
 }
 
-export function brushUnavailableHint(document: ImageDocument, editor: EditorState): string | undefined {
+export function brushUnavailableHint(document: ImageDocument, editor: BrushTargetEditorState): string | undefined {
   if (isPanInteractionMode(editor.tools.interactionMode) || (editor.tools.activeTool !== "brush" && editor.tools.activeTool !== "eraser")) return undefined;
   if (resolveBrushTargetLayer(document, editor)) return undefined;
 
@@ -46,7 +52,7 @@ export function brushUnavailableHint(document: ImageDocument, editor: EditorStat
   return "Select a raster layer or layer mask to paint.";
 }
 
-function resolveBrushTargetLayer(document: ImageDocument, editor: EditorState): RasterLayer | undefined {
+function resolveBrushTargetLayer(document: ImageDocument, editor: BrushTargetEditorState): RasterLayer | undefined {
   if (isPanInteractionMode(editor.tools.interactionMode) || (editor.tools.activeTool !== "brush" && editor.tools.activeTool !== "eraser")) return undefined;
   const editingMask = Boolean(editor.maskEdit);
   const layerId = editor.maskEdit?.maskLayerId ?? editor.selection.layerIds[0];
diff --git a/view/generate/runGenerate.ts b/view/generate/runGenerate.ts
index b8b7384..852303e 100644
--- a/view/generate/runGenerate.ts
+++ b/view/generate/runGenerate.ts
@@ -12,7 +12,7 @@ export async function runGenerate(options: {
   settings: GenerateSettings;
   dispatch: AppStore["dispatch"];
 }) {
-  const { document, selection, viewport, settings, dispatch } = options;
+  const { document, selection, settings, dispatch } = options;
   const artboard = selection.artboardId ? document.artboards.find((candidate) => candidate.id === selection.artboardId) : document.artboards[0];
   if (!artboard) return;
 
diff --git a/view/useAppState.test.ts b/view/useAppState.test.ts
new file mode 100644
index 0000000..29383c7
--- /dev/null
+++ b/view/useAppState.test.ts
@@ -0,0 +1,68 @@
+import { describe, expect, test } from "bun:test";
+import { createInitialAppState } from "@editor/initial-state";
+import type { AppState } from "@editor/state";
+import { createAppStateSelectorSnapshot, shallowEqual } from "./useAppState";
+
+describe("app state selector snapshot", () => {
+  test("reuses selected snapshots when equality reports no relevant changes", () => {
+    const state = createInitialAppState("Test");
+    const snapshot = createAppStateSelectorSnapshot(selectShellLikeState, shallowEqual);
+    const selected = snapshot.getSnapshot(state);
+    const previewOnlyUpdate: AppState = {
+      ...state,
+      editor: {
+        ...state.editor,
+        brushStrokePreview: { layerId: "layer", assetId: "asset", source: "preview" },
+      },
+    };
+
+    expect(snapshot.updateSnapshot(previewOnlyUpdate)).toBe(false);
+    expect(snapshot.getSnapshot(previewOnlyUpdate)).toBe(selected);
+  });
+
+  test("can collapse brush preview updates to presence instead of position", () => {
+    const state = createInitialAppState("Test");
+    const snapshot = createAppStateSelectorSnapshot((next: AppState) => ({ hasBrushPreview: Boolean(next.editor.brushPreview) }), shallowEqual);
+    const withoutPreview = snapshot.getSnapshot(state);
+    const withPreview: AppState = {
+      ...state,
+      editor: {
+        ...state.editor,
+        brushPreview: { position: { x: 10, y: 20 } },
+      },
+    };
+
+    expect(snapshot.updateSnapshot(withPreview)).toBe(true);
+    const previewPresent = snapshot.getSnapshot(withPreview);
+    expect(previewPresent).not.toBe(withoutPreview);
+    expect(previewPresent.hasBrushPreview).toBe(true);
+
+    const movedPreview: AppState = {
+      ...withPreview,
+      editor: {
+        ...withPreview.editor,
+        brushPreview: { position: { x: 30, y: 40 } },
+      },
+    };
+
+    expect(snapshot.updateSnapshot(movedPreview)).toBe(false);
+    expect(snapshot.getSnapshot(movedPreview)).toBe(previewPresent);
+  });
+
+  test("shallow equality uses object keys and Object.is value checks", () => {
+    expect(shallowEqual({ id: "a", value: Number.NaN }, { id: "a", value: Number.NaN })).toBe(true);
+    expect(shallowEqual({ id: "a", value: 0 }, { id: "a", value: -0 })).toBe(false);
+    expect(shallowEqual({ id: "a" }, { id: "a", extra: true })).toBe(false);
+  });
+});
+
+function selectShellLikeState(state: AppState) {
+  return {
+    document: state.document,
+    selection: state.editor.selection,
+    viewport: state.editor.viewport,
+    tools: state.editor.tools,
+    transformSession: state.editor.transformSession,
+    maskEdit: state.editor.maskEdit,
+  };
+}
diff --git a/view/useAppState.ts b/view/useAppState.ts
index e5e035d..9ce5544 100644
--- a/view/useAppState.ts
+++ b/view/useAppState.ts
@@ -1,7 +1,102 @@
-import { useSyncExternalStore } from "react";
+import { useCallback, useRef, useSyncExternalStore } from "react";
 import type { AppStore } from "@editor/store";
 import type { AppState } from "@editor/state";
 
-export function useAppState(store: AppStore): AppState {
-  return useSyncExternalStore(store.subscribe, store.getState, store.getState);
+export type AppStateSelector<TSelection> = (state: AppState) => TSelection;
+export type AppStateEquality<TSelection> = (a: TSelection, b: TSelection) => boolean;
+
+type AppStateSelectorSnapshot<TSelection> = {
+  getSnapshot(state: AppState): TSelection;
+  updateSnapshot(state: AppState): boolean;
+};
+
+type AppStateSelectorRef<TSelection> = {
+  selector: AppStateSelector<TSelection>;
+  isEqual: AppStateEquality<TSelection>;
+  snapshot: AppStateSelectorSnapshot<TSelection>;
+};
+
+const selectAppState = (state: AppState) => state;
+
+export function useAppState(store: AppStore): AppState;
+export function useAppState<TSelection>(
+  store: AppStore,
+  selector: AppStateSelector<TSelection>,
+  isEqual?: AppStateEquality<TSelection>,
+): TSelection;
+export function useAppState<TSelection>(
+  store: AppStore,
+  selector: AppStateSelector<TSelection> = selectAppState as AppStateSelector<TSelection>,
+  isEqual: AppStateEquality<TSelection> = Object.is,
+): TSelection {
+  const selectorRef = useRef<AppStateSelectorRef<TSelection> | undefined>(undefined);
+
+  if (!selectorRef.current || selectorRef.current.selector !== selector || selectorRef.current.isEqual !== isEqual) {
+    selectorRef.current = {
+      selector,
+      isEqual,
+      snapshot: createAppStateSelectorSnapshot(selector, isEqual),
+    };
+  }
+
+  const getSnapshot = useCallback(() => selectorRef.current!.snapshot.getSnapshot(store.getState()), [store]);
+  const subscribe = useCallback(
+    (onStoreChange: () => void) =>
+      store.subscribe((state) => {
+        if (selectorRef.current!.snapshot.updateSnapshot(state)) onStoreChange();
+      }),
+    [store],
+  );
+
+  return useSyncExternalStore(subscribe, getSnapshot, getSnapshot);
+}
+
+export function createAppStateSelectorSnapshot<TSelection>(
+  selector: AppStateSelector<TSelection>,
+  isEqual: AppStateEquality<TSelection> = Object.is,
+): AppStateSelectorSnapshot<TSelection> {
+  let stateSnapshot: AppState | undefined;
+  let selectedSnapshot: TSelection | undefined;
+  let hasSnapshot = false;
+
+  const setSnapshot = (state: AppState, selected: TSelection) => {
+    stateSnapshot = state;
+    selectedSnapshot = selected;
+    hasSnapshot = true;
+  };
+
+  return {
+    getSnapshot(state) {
+      if (hasSnapshot && stateSnapshot === state) return selectedSnapshot as TSelection;
+
+      const selected = selector(state);
+      if (hasSnapshot && isEqual(selectedSnapshot as TSelection, selected)) {
+        stateSnapshot = state;
+        return selectedSnapshot as TSelection;
+      }
+
+      setSnapshot(state, selected);
+      return selected;
+    },
+    updateSnapshot(state) {
+      const selected = selector(state);
+      if (hasSnapshot && isEqual(selectedSnapshot as TSelection, selected)) {
+        stateSnapshot = state;
+        return false;
+      }
+
+      setSnapshot(state, selected);
+      return true;
+    },
+  };
+}
+
+export function shallowEqual<TObject extends object>(a: TObject, b: TObject): boolean {
+  if (Object.is(a, b)) return true;
+
+  const aKeys = Object.keys(a) as Array<keyof TObject>;
+  const bKeys = Object.keys(b) as Array<keyof TObject>;
+  if (aKeys.length !== bKeys.length) return false;
+
+  return aKeys.every((key) => Object.prototype.hasOwnProperty.call(b, key) && Object.is(a[key], b[key]));
 }