Show a wheel step or Fit as a short zoom transition (#309)

The wheel and the Fit button used to jump to the new view. Now the
view takes a short way there, 200 ms by default, everywhere the editor
zooms. The zoom steps stay the same, only the change is shown.

- viewTween: tweenView is the way between two views as a pure
  function. The scale moves by equal ratios, and the world point that
  sits at the same place on screen in both views stays there. After a
  wheel step that is the point under the pointer.
- ViewTween drives it on the ticker. A surface hands it reading,
  writing and drawing its own view and wraps its wheel and Fit code in
  change(). A step during a transition starts from where that one was
  heading, so quick steps add up to exactly as many steps.
- Anything else that moves the view ends the transition. A pan jumps
  to the target first, and saving a view saves the target.
- Wired into FreeViewport (Scene, Container, animation preview),
  SheetCanvas (Texture, Curve, Pixel art, Vector art), GraphCanvas,
  panAndZoom (Tile board, plugin node board) and the timeline.
- Menu > General settings... sets the time in ms, 0 jumps at once. It
  is kept in this browser like the panel sizes, not in the project,
  and it is not an undo step.
- A document that opens still comes up framed at once. The animation
  preview frames a new subject itself instead of calling fit().
- The pixel art readout names the step the view is heading for.

Measured in the scene view at 200 ms on melektaus-atx, where every
frame of the transition is a full redraw:

- 500 sprites: redraw 6.7 ms, 16 frames, mean 18 ms
- 1000 sprites: redraw 13.0 ms, 9 frames, mean 27 ms
- 2000 sprites: redraw 32.2 ms, 5 frames, mean 58 ms

Tests:
- The harness sets 0 ms on every load, so the existing modules read
  the step and not the way.
- e2e 46i samples one wheel step at 1000 ms a frame later: zoom 1.033
  between 1 and 2, the point under the pointer kept at (134, -40). It
  lands on the view a 0 ms jump reaches.
- e2e 46 and 49 expect the new menu entry.
- Gate (atx): 234 files, 3563 tests, CHECK EXIT 0 (+15)
- e2e 46,49 (atx): ALL PASSED, 43 checks (+3, sum 1735)
- Three counter-checks fail view-tween.test.ts as expected. e2e 46i
  fails when the scene view does not carry its centre along.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/Editor.ts

@@ -45,6 +45,8 @@ import type { ReleaseStore } from '../core/store/ReleaseStore';
 import type { ReleaseDialog } from './ReleaseDialog';
 import type { ProjectSettingsDialog } from './ProjectSettingsDialog';
 import { dialogSlot } from './FormDialog';
+import { generalSettings, keepGeneralSettings } from './generalSettings';
+import { GeneralSettingsDialog } from './GeneralSettingsDialog';
 import { ContainerEditor } from './container/ContainerEditor';
 import { EditorRun } from './EditorRun';
 import { GraphEditor } from './graph/GraphEditor';
@@ -256,6 +258,7 @@ export class Editor implements PlayHost {
   plugins: PluginManager | null = null;
   private readonly toolbar: Toolbar;
   private readonly passwordDialog = dialogSlot<PasswordDialog>();
+  private readonly generalDialog = dialogSlot<GeneralSettingsDialog>();
   /** Drawn-frame count at the last measurement, not a counter of its own. */
   private fpsFrames = 0;
   private fpsElapsed = 0;
@@ -601,11 +604,22 @@ export class Editor implements PlayHost {
     const open = () => this.openDocument('management');
     return [
       { separator: true },
+      { label: 'General settings…', onSelect: () => this.openGeneralSettings() },
       ...(this.usersPane ? [{ label: 'User management', onSelect: open }] : []),
       { label: 'Plugins', onSelect: open },
     ];
   }
 
+  /** What the editor does for whoever sits at it, kept in this browser (#309). */
+  openGeneralSettings(): GeneralSettingsDialog {
+    return this.generalDialog.open(new GeneralSettingsDialog(generalSettings(), keepGeneralSettings));
+  }
+
+  /** The open "General settings" dialog, for the check scripts. */
+  generalSettingsDialog(): GeneralSettingsDialog | null {
+    return this.generalDialog.current();
+  }
+
   /**
    * The account menu under its button (#182). The name stands in it as an entry that cannot be
    * picked: it says who is signed in without costing the crowded toolbar another label.
@@ -921,6 +935,7 @@ export class Editor implements PlayHost {
     for (const cleanup of this.cleanups.splice(0).reverse()) cleanup();
     // Closed rather than just thrown away with the overlay below, so the slot lets go of it too.
     this.passwordDialog.current()?.close();
+    this.generalDialog.current()?.close();
     // The tooltip is not the editor's to destroy: it leaves the overlay before that is emptied.
     tooltips.dismiss();
     // A menu or a picker left open belongs to widgets that are about to be destroyed.

src/editor/GeneralSettingsDialog.ts

@@ -0,0 +1,45 @@
+import { Label, NumberField } from '../ui';
+import { type GeneralSettings, ZOOM_TRANSITION_MAX_MS, zoomTransitionMs } from './generalSettings';
+import { FormDialog } from './FormDialog';
+
+/**
+ * How the editor behaves for whoever sits at it (#309), as opposed to the project's own settings:
+ * kept in this browser, the same in every project, and not an undo step.
+ */
+export class GeneralSettingsDialog extends FormDialog {
+  private zoomMs: number;
+  private readonly zoomField: NumberField;
+
+  constructor(settings: GeneralSettings, onApply: (settings: GeneralSettings) => void) {
+    super(340, 'General settings');
+    this.zoomMs = settings.zoomTransitionMs;
+    this.zoomField = new NumberField(this.zoomMs, {
+      width: 150,
+      min: 0,
+      max: ZOOM_TRANSITION_MAX_MS,
+      step: 10,
+      decimals: 0,
+      onInput: (value) => {
+        this.zoomMs = value;
+      },
+      onChange: (value) => {
+        this.zoomMs = value;
+      },
+    });
+    this.content.add(this.formRow(this.fieldLabel('Zoom (ms)'), this.zoomField));
+    this.content.add(
+      new Label('How long a zoom or Fit takes to settle. 0 jumps at once.', { style: 'muted' }),
+    );
+    this.addButtons('Apply', 'primary', () => onApply({ zoomTransitionMs: zoomTransitionMs(this.zoomMs) }));
+  }
+
+  /** The settings as the fields stand (test hook). */
+  values(): GeneralSettings {
+    return { zoomTransitionMs: this.zoomMs };
+  }
+
+  /** The zoom field itself, so a check can type into it. */
+  get zoomControl(): NumberField {
+    return this.zoomField;
+  }
+}

src/editor/animation/PreviewCanvas.ts

@@ -63,12 +63,15 @@ export class PreviewCanvas extends FreeViewport {
     return item ? { x: item.x, y: item.y, props: item.props } : null;
   }
 
-  /** Skipped while the tab is hidden, like the Game tab: a frame costs a collect of the root. */
+  /**
+   * Skipped while the tab is hidden, like the Game tab: a frame costs a collect of the root. A fresh
+   * subject is framed at once, not by a transition from the last one's view.
+   */
   override redraw(): void {
     if (!this.visible) return;
     if (this.fitPending && this.w > 0 && this.h > 0) {
-      this.fit();
-      return;
+      this.fitPending = false;
+      this.frameContent();
     }
     super.redraw();
   }

src/editor/animation/TimelineCanvas.ts

@@ -43,6 +43,7 @@ import {
   crispText,
 } from '../../ui';
 import { type EditorState, trackSelection } from '../state';
+import { ViewTween } from '../viewTween';
 import { FramesDialog, openFramesDialog } from './FramesDialog';
 import {
   clampMove,
@@ -107,6 +108,14 @@ export class TimelineCanvas extends Widget {
   private readonly hint = crispText({ text: '', style: textStyle('muted') });
   private readonly clip = new Graphics();
   private view: TimelineView = { ...DEFAULT_VIEW };
+  /** Only across: a frame's cell is `cellWidth` wide and frame 0 starts at `panX`. */
+  private readonly tween = new ViewTween({
+    read: () => ({ scale: this.view.cellWidth, x: this.view.panX, y: 0 }),
+    write: (map) => {
+      this.view = { ...this.view, panX: map.x, cellWidth: map.scale };
+    },
+    draw: () => this.redraw(),
+  });
   private assetId = '';
   /** Where the playhead stands, in frames – fractional while playing. */
   playhead = 0;
@@ -163,6 +172,7 @@ export class TimelineCanvas extends Widget {
   }
 
   saveView(): TimelineView {
+    this.tween.land();
     return { ...this.view };
   }
 
@@ -403,6 +413,7 @@ export class TimelineCanvas extends Widget {
     const local = this.toLocal(e.global);
     const point = { x: local.x, y: local.y };
     if (e.button === 1 || (e.button === 0 && e.shiftKey && local.x >= HEADER_WIDTH)) {
+      this.tween.finish();
       this.panning = { pointer: point, panX: this.view.panX };
       e.stopPropagation();
       return;
@@ -529,11 +540,15 @@ export class TimelineCanvas extends Widget {
   private onWheel(e: FederatedWheelEvent): void {
     const local = this.toLocal(e.global);
     if (e.shiftKey) {
+      this.tween.land();
       this.view = clampPan({ ...this.view, panX: this.view.panX - e.deltaY });
+      this.redraw();
     } else {
-      this.view = clampPan(zoomCells(this.view, zoomFactor(e.deltaY), local.x - HEADER_WIDTH));
+      this.tween.change(() => {
+        this.view = clampPan(zoomCells(this.view, zoomFactor(e.deltaY), local.x - HEADER_WIDTH));
+        this.redraw();
+      });
     }
-    this.redraw();
     e.preventDefault();
   }
 

src/editor/debug/sceneHooks.ts

@@ -51,6 +51,10 @@ export function sceneHooks({ state, editor }: HookContext) {
     sceneHeaderAt: (name: 'fit' | 'visibility') =>
       centreOf(name === 'fit' ? scenePane.fitButton : scenePane.visibilityButton),
     viewportInfo: () => viewport.debugInfo(),
+    /** The scene view's centre and zoom, and whether a zoom transition is under way (#309). */
+    sceneView: () => viewport.viewState(),
+    /** The world point under a screen point, as the scene view shows it right now. */
+    sceneWorldAt: (x: number, y: number) => viewport.worldAt(x, y),
     /** A full redraw of the Scene tab – what a pan, which only moves the camera, is compared with. */
     viewportRedraw: () => viewport.redraw(),
     /** Where a dragged asset would land in the Scene tab, and why not when it may not. */

src/editor/debug/uiHooks.ts

@@ -13,6 +13,7 @@ import {
 } from '../../ui';
 import { Theme } from '../../ui/Theme';
 import { currentFontPicker } from '../font/FontPicker';
+import { generalSettings, keepGeneralSettings } from '../generalSettings';
 import { currentGroupDialog } from '../panels/GroupNameDialog';
 import { currentSoundPicker } from '../sound/SoundPicker';
 import { rectOf, textProbe } from '../textProbe';
@@ -318,6 +319,21 @@ export function uiHooks({ app, state, editor }: HookContext) {
       const dialog = editor.projectSettingsDialog();
       return dialog ? centreOf(dialog.symbolScaleControl) : null;
     },
+    /** The "General settings" dialog (#309) as its field stands, null while shut. */
+    generalSettingsDialog: () => editor.generalSettingsDialog()?.values() ?? null,
+    /** Where to click the zoom field inside that dialog. */
+    generalSettingsZoomAt: () => {
+      const dialog = editor.generalSettingsDialog();
+      return dialog ? centreOf(dialog.zoomControl) : null;
+    },
+    /**
+     * The zoom transition in ms as kept in this browser; with `ms` it is set first. The harness sets
+     * 0 on every load, so a check reading the zoom after the wheel finds the step and not the way.
+     */
+    zoomTransition: (ms?: number) => {
+      if (ms !== undefined) keepGeneralSettings({ zoomTransitionMs: ms });
+      return generalSettings().zoomTransitionMs;
+    },
     /** The "Open project" dialog (#183): the projects it lists and the row picked. */
     projectPicker: () => editor.projectPickerDialog()?.info() ?? null,
     /** The "Snapshots" dialog (#231): its rows, newest first, the row picked and its name field. */

src/editor/generalSettings.ts

@@ -0,0 +1,64 @@
+/**
+ * How the editor itself behaves, kept in this browser rather than in a project (#309): it is about
+ * the person at the screen, not the game. Best effort, like the panel sizes – a browser that
+ * refuses `localStorage` keeps the settings for the page only.
+ */
+import { clamp } from '../ui/math';
+
+const STORAGE_KEY = 'game-maker.general-settings';
+
+export interface GeneralSettings {
+  /** How long a zoom takes to reach its new step, in ms; 0 jumps at once. */
+  zoomTransitionMs: number;
+}
+
+export const ZOOM_TRANSITION_MAX_MS = 1000;
+
+const DEFAULTS: GeneralSettings = { zoomTransitionMs: 200 };
+
+let settings: GeneralSettings | null = null;
+
+function storage(): Pick<Storage, 'getItem' | 'setItem'> | null {
+  try {
+    return typeof window === 'undefined' ? null : window.localStorage;
+  } catch {
+    return null;
+  }
+}
+
+/** A whole number of ms within the range the dialog offers; anything else is the default. */
+export function zoomTransitionMs(value: unknown): number {
+  return typeof value === 'number' && Number.isFinite(value)
+    ? Math.round(clamp(value, 0, ZOOM_TRANSITION_MAX_MS))
+    : DEFAULTS.zoomTransitionMs;
+}
+
+function read(): GeneralSettings {
+  try {
+    const stored: unknown = JSON.parse(storage()?.getItem(STORAGE_KEY) ?? '{}');
+    const raw = typeof stored === 'object' && stored !== null ? (stored as Record<string, unknown>) : {};
+    return { zoomTransitionMs: zoomTransitionMs(raw.zoomTransitionMs) };
+  } catch {
+    return { ...DEFAULTS };
+  }
+}
+
+export function generalSettings(): GeneralSettings {
+  settings ??= read();
+  return { ...settings };
+}
+
+export function keepGeneralSettings(change: Partial<GeneralSettings>): void {
+  settings = { ...generalSettings(), ...change };
+  settings.zoomTransitionMs = zoomTransitionMs(settings.zoomTransitionMs);
+  try {
+    storage()?.setItem(STORAGE_KEY, JSON.stringify(settings));
+  } catch {
+    // Kept for this page only.
+  }
+}
+
+/** Forgets what was read, so the next question asks the browser again (tests). */
+export function resetGeneralSettings(): void {
+  settings = null;
+}

src/editor/graph/GraphCanvas.ts

@@ -30,6 +30,7 @@ import { particleLibrary } from '../../viewport/ParticleLibrary';
 import { shaderLibrary } from '../../viewport/ShaderLibrary';
 import { findAsset, findGraphAsset } from '../../core/model/Project';
 import { type Trace, TRACE_FADE } from '../../runtime/systems/BehaviorSystem';
+import { ViewTween } from '../viewTween';
 import { BeadClock } from './beads';
 import {
   activeDrag,
@@ -116,6 +117,14 @@ export class GraphCanvas extends Widget {
   private assetId = '';
   private pan = { x: 0, y: 0 };
   private zoom = 1;
+  private readonly tween = new ViewTween({
+    read: () => ({ scale: this.zoom, x: this.pan.x, y: this.pan.y }),
+    write: (map) => {
+      this.pan = { x: map.x, y: map.y };
+      this.zoom = map.scale;
+    },
+    draw: () => this.applyView(),
+  });
   private panning: { startX: number; startY: number; panX: number; panY: number } | null = null;
   private snapping = false;
   private readonly wiring: GraphWiring;
@@ -190,12 +199,15 @@ export class GraphCanvas extends Widget {
     this.on('pointerup', (e: FederatedPointerEvent) => this.onPointerUp(e));
     this.on('pointerupoutside', (e: FederatedPointerEvent) => this.onPointerUp(e));
     this.on('wheel', (e: FederatedWheelEvent) => {
-      ({ pan: this.pan, zoom: this.zoom } = zoomedAbout(
-        { pan: this.pan, zoom: this.zoom },
-        this.toLocal(e.global),
-        zoomFactor(e.deltaY),
-      ));
-      this.applyView();
+      const at = this.toLocal(e.global);
+      this.tween.change(() => {
+        ({ pan: this.pan, zoom: this.zoom } = zoomedAbout(
+          { pan: this.pan, zoom: this.zoom },
+          at,
+          zoomFactor(e.deltaY),
+        ));
+        this.applyView();
+      });
       e.preventDefault();
     });
 
@@ -226,11 +238,14 @@ export class GraphCanvas extends Widget {
   }
 
   fit(): void {
-    this.needsFit = true;
-    this.fitView();
+    this.tween.change(() => {
+      this.needsFit = true;
+      this.fitView();
+    });
   }
 
   saveView(): GraphViewState {
+    this.tween.land();
     return { pan: { ...this.pan }, zoom: this.zoom, needsFit: this.needsFit };
   }
 
@@ -507,6 +522,7 @@ export class GraphCanvas extends Widget {
     // The middle button pans from anywhere, a node under the pointer included – it is the one
     // gesture that never means "grab this". Nodes let it through for exactly that reason.
     const startPan = () => {
+      this.tween.finish();
       this.panning = { startX: e.global.x, startY: e.global.y, panX: this.pan.x, panY: this.pan.y };
     };
     if (e.button === 1) {

src/editor/graph/graphView.ts

@@ -1,6 +1,7 @@
 import type { Container, FederatedPointerEvent, FederatedWheelEvent, Graphics } from 'pixi.js';
 import { clamp, drawGrid, zoomFactor } from '../../ui';
 import type { Point } from '../framing';
+import { ViewTween } from '../viewTween';
 
 /**
  * What every surface that shows nodes shares with the graph editor: how far it zooms, how the grid
@@ -48,18 +49,34 @@ export function zoomedAbout(
 export type WheelZoom =
   { min: number; max: number } | ((view: GraphView, at: Point, deltaY: number) => GraphView | null);
 
+/** The transition (#309) of a board whose view is a `GraphView`. */
+export function graphViewTween(
+  get: () => GraphView,
+  put: (view: GraphView) => void,
+  draw: () => void,
+): ViewTween {
+  return new ViewTween({
+    read: () => ({ scale: get().zoom, x: get().pan.x, y: get().pan.y }),
+    write: (map) => put({ pan: { x: map.x, y: map.y }, zoom: map.scale }),
+    draw,
+  });
+}
+
 /**
  * Drag with any button to pan, wheel to zoom about the pointer – all a board does with the pointer
- * when its content takes none. `set` receives each new view and which gesture made it.
+ * when its content takes none. `set` receives each new view and which gesture made it; `tween`
+ * shows the wheel's views as a transition (#309).
  */
 export function panAndZoom(
   surface: Container,
   get: () => GraphView,
   set: (view: GraphView, gesture: 'pan' | 'zoom') => void,
   zoom: WheelZoom = ZOOM_LIMITS,
+  tween: ViewTween,
 ): void {
   let panning: { start: Point; pan: Point } | null = null;
   surface.on('pointerdown', (e: FederatedPointerEvent) => {
+    tween.finish();
     panning = { start: { x: e.global.x, y: e.global.y }, pan: { ...get().pan } };
   });
   surface.on('globalpointermove', (e: FederatedPointerEvent) => {
@@ -72,11 +89,13 @@ export function panAndZoom(
   surface.on('wheel', (e: FederatedWheelEvent) => {
     e.preventDefault();
     const at = surface.toLocal(e.global);
-    const view =
-      typeof zoom === 'function'
-        ? zoom(get(), at, e.deltaY)
-        : zoomedAbout(get(), at, zoomFactor(e.deltaY), zoom);
-    if (view) set(view, 'zoom');
+    tween.change(() => {
+      const view =
+        typeof zoom === 'function'
+          ? zoom(get(), at, e.deltaY)
+          : zoomedAbout(get(), at, zoomFactor(e.deltaY), zoom);
+      if (view) set(view, 'zoom');
+    });
   });
 }
 

src/editor/pixelart/PixelArtCanvas.ts

@@ -151,8 +151,9 @@ export class PixelArtCanvas extends SheetCanvas<PixelArtAsset> {
     this.onChange();
   }
 
+  /** The step the view is at, or heading for during a transition – the readout names steps only. */
   get zoom(): number {
-    return this.view.zoom;
+    return this.tween.heading?.scale ?? this.view.zoom;
   }
 
   asset(): PixelArtAsset | null {
@@ -354,8 +355,9 @@ export class PixelArtCanvas extends SheetCanvas<PixelArtAsset> {
 
   protected override onWheel(e: FederatedWheelEvent): void {
     const local = this.toLocal(e.global);
-    const zoom = stepZoom(this.view.zoom, e.deltaY < 0 ? 1 : -1);
-    if (zoom !== this.view.zoom) {
+    this.tween.change(() => {
+      const zoom = stepZoom(this.view.zoom, e.deltaY < 0 ? 1 : -1);
+      if (zoom === this.view.zoom) return;
       // The pixel under the pointer stays under it.
       const x = (local.x - this.view.panX) / this.view.zoom;
       const y = (local.y - this.view.panY) / this.view.zoom;
@@ -363,7 +365,7 @@ export class PixelArtCanvas extends SheetCanvas<PixelArtAsset> {
       this.needsFit = false;
       this.redraw();
       this.onChange();
-    }
+    });
     e.preventDefault();
   }
 

src/editor/plugins/NodeBoard.ts

@@ -2,7 +2,14 @@ import { Container, Graphics } from 'pixi.js';
 import type { NodeDef } from '../../core/graph';
 import { Button, Label, type Size, Spacer, Theme, VStack, Widget } from '../../ui';
 import { type Box, frameBoxes } from '../framing';
-import { drawGraphGrid, FIT_MARGIN, type GraphView, panAndZoom, ZOOM_LIMITS } from '../graph/graphView';
+import {
+  drawGraphGrid,
+  FIT_MARGIN,
+  type GraphView,
+  graphViewTween,
+  panAndZoom,
+  ZOOM_LIMITS,
+} from '../graph/graphView';
 import { NodeView } from '../graph/NodeView';
 import { TabHeader } from '../TabHeader';
 
@@ -53,6 +60,11 @@ class BoardCanvas extends Widget {
   private view: GraphView = { pan: { x: 0, y: 0 }, zoom: 1 };
   /** Framed once there is a size and something to frame; after that the view is the user's. */
   private needsFit = true;
+  private readonly tween = graphViewTween(
+    () => this.view,
+    (view) => (this.view = view),
+    () => this.applyView(),
+  );
 
   constructor() {
     super();
@@ -66,6 +78,8 @@ class BoardCanvas extends Widget {
         this.view = view;
         this.applyView();
       },
+      ZOOM_LIMITS,
+      this.tween,
     );
   }
 
@@ -84,6 +98,11 @@ class BoardCanvas extends Widget {
     else this.applyView();
   }
 
+  /** Fit by hand: the Fit button and a plugin's call, shown as a transition. */
+  refit(): void {
+    this.tween.change(() => this.fit());
+  }
+
   fit(): void {
     if (this.w === 0 || this.h === 0) return;
     const boxes: Box[] = this.views.map((v) => ({ id: v.node.id, x: v.x, y: v.y, w: v.w, h: v.h }));
@@ -151,7 +170,7 @@ export class NodeBoard extends VStack {
   }
 
   fit(): void {
-    this.canvas.fit();
+    this.canvas.refit();
   }
 
   /** The nodes as drawn and the view – what the check scripts read. */

src/editor/texture/SheetCanvas.ts

@@ -4,6 +4,7 @@ import type { Vec2 } from '../../core/model/Properties';
 import type { CutSize } from '../../core/model/texture';
 import { type Size, Theme, textStyle, ui, Widget, zoomFactor, crispText } from '../../ui';
 import { dashedLine } from '../../viewport/symbols';
+import { ViewTween } from '../viewTween';
 import { cutLines, imageToLocal, type SheetLine, type SheetView, zoomAbout } from './sheetView';
 
 /**
@@ -24,6 +25,13 @@ export abstract class SheetCanvas<A> extends Widget {
   /** Fit once the widget has a real size and a picture to fit (a tab starts hidden). */
   protected needsFit = true;
   private panning: { pointer: Vec2; panX: number; panY: number } | null = null;
+  protected readonly tween = new ViewTween({
+    read: () => ({ scale: this.view.zoom, x: this.view.panX, y: this.view.panY }),
+    write: (map) => {
+      this.view = { panX: map.x, panY: map.y, zoom: map.scale };
+    },
+    draw: () => this.redraw(),
+  });
 
   constructor() {
     super();
@@ -66,6 +74,7 @@ export abstract class SheetCanvas<A> extends Widget {
   }
 
   saveView(): { view: SheetView; needsFit: boolean } {
+    this.tween.land();
     return { view: { ...this.view }, needsFit: this.needsFit };
   }
 
@@ -78,8 +87,10 @@ export abstract class SheetCanvas<A> extends Widget {
   }
 
   fit(): void {
-    this.needsFit = true;
-    this.redraw();
+    this.tween.change(() => {
+      this.needsFit = true;
+      this.redraw();
+    });
   }
 
   /** Where a picture pixel sits on screen; the check scripts aim their drags with it. */
@@ -96,9 +107,11 @@ export abstract class SheetCanvas<A> extends Widget {
 
   protected onWheel(e: FederatedWheelEvent): void {
     const local = this.toLocal(e.global);
-    this.view = zoomAbout(this.view, zoomFactor(e.deltaY), local.x, local.y);
-    this.needsFit = false;
-    this.redraw();
+    this.tween.change(() => {
+      this.view = zoomAbout(this.view, zoomFactor(e.deltaY), local.x, local.y);
+      this.needsFit = false;
+      this.redraw();
+    });
     e.preventDefault();
   }
 
@@ -129,6 +142,7 @@ export abstract class SheetCanvas<A> extends Widget {
     const local = this.toLocal(e.global);
     const pans = e.button === 0 && e.shiftKey && !this.shiftPresses(target, { x: local.x, y: local.y });
     if (e.button === 1 || pans) {
+      this.tween.finish();
       this.panning = { pointer: { x: local.x, y: local.y }, panX: this.view.panX, panY: this.view.panY };
       e.stopPropagation();
       return;

src/editor/tile/TileCanvas.ts

@@ -23,7 +23,7 @@ import { type Size, Theme, textStyle, ui, Widget, crispText } from '../../ui';
 import { textureLibrary } from '../../viewport/TextureLibrary';
 import { nearestZoomStep, stepFreeZoom, wheelSteps, zoomStepBelow } from '../../viewport/view';
 import type { Point } from '../framing';
-import { type GraphView, panAndZoom, zoomedAbout } from '../graph/graphView';
+import { type GraphView, graphViewTween, panAndZoom, zoomedAbout } from '../graph/graphView';
 import type { EditorState } from '../state';
 
 /** Tiles per row in the combination grid; 47 of them then read as six rows of eight. */
@@ -108,6 +108,11 @@ export class TileCanvas extends Widget {
   private fittedFor = { w: 0, h: 0, board: { w: 0, h: 0 } };
   private readonly stopPictures: () => void;
   private readonly wheel = wheelSteps();
+  private readonly tween = graphViewTween(
+    () => this.view,
+    (view) => (this.view = view),
+    () => this.applyView(),
+  );
 
   constructor(private readonly state: EditorState) {
     super();
@@ -128,6 +133,7 @@ export class TileCanvas extends Widget {
         this.applyView();
       },
       (view, at, deltaY) => this.zoomedStep(view, at, deltaY),
+      this.tween,
     );
 
     state.on('project', () => this.redraw());
@@ -163,12 +169,15 @@ export class TileCanvas extends Widget {
   }
 
   fit(): void {
-    this.needsFit = true;
-    this.followsFit = true;
-    this.redraw();
+    this.tween.change(() => {
+      this.needsFit = true;
+      this.followsFit = true;
+      this.redraw();
+    });
   }
 
   saveView(): { view: GraphView; needsFit: boolean } {
+    this.tween.land();
     return { view: { pan: { ...this.view.pan }, zoom: this.view.zoom }, needsFit: this.needsFit };
   }
 

src/editor/viewTween.ts

@@ -0,0 +1,156 @@
+import { requestRender, ui } from '../ui';
+import { generalSettings } from './generalSettings';
+
+/**
+ * A view as every zoomable surface of the editor has one, whatever its fields are called: a world
+ * point `p` is shown at `p * scale + (x, y)`.
+ */
+export interface ViewMap {
+  scale: number;
+  x: number;
+  y: number;
+}
+
+/** Quick at first and gentle at the end: the zoom answers the wheel at once and settles softly. */
+export function easeOut(t: number): number {
+  const rest = 1 - Math.min(1, Math.max(0, t));
+  return 1 - rest * rest * rest;
+}
+
+/**
+ * The view `t` of the way from `from` to `to` (#309). The scale moves by equal ratios, so zooming
+ * in and out feel equally fast. The world point that sits at the same place on screen in both
+ * views stays there all the way – after a wheel step that is the point under the pointer. Two
+ * views of the same scale have no such point, and between them the view slides.
+ */
+export function tweenView(from: ViewMap, to: ViewMap, t: number): ViewMap {
+  if (t <= 0) return { ...from };
+  if (t >= 1) return { ...to };
+  const scale = from.scale * (to.scale / from.scale) ** t;
+  const gap = from.scale - to.scale;
+  if (Math.abs(gap) <= 1e-9 * from.scale)
+    return { scale, x: from.x + (to.x - from.x) * t, y: from.y + (to.y - from.y) * t };
+  const held = (a: number, b: number) => {
+    const world = (b - a) / gap;
+    return a + world * from.scale - world * scale;
+  };
+  return { scale, x: held(from.x, to.x), y: held(from.y, to.y) };
+}
+
+function near(a: ViewMap, b: ViewMap): boolean {
+  const close = (p: number, q: number) => Math.abs(p - q) <= 1e-6 * Math.max(1, Math.abs(p), Math.abs(q));
+  return close(a.scale, b.scale) && close(a.x, b.x) && close(a.y, b.y);
+}
+
+/** How a surface hands its view to the tween. */
+export interface TweenedView {
+  read(): ViewMap;
+  /** Puts a view in place without drawing it. */
+  write(view: ViewMap): void;
+  draw(): void;
+}
+
+/** Time and frames; the editor's are the ticker's, a test brings its own. */
+export interface TweenClock {
+  now(): number;
+  add(fn: () => void): void;
+  remove(fn: () => void): void;
+}
+
+const tickerClock: TweenClock = {
+  now: () => performance.now(),
+  add: (fn) => ui().app.ticker.add(fn),
+  remove: (fn) => ui().app.ticker.remove(fn),
+};
+
+/**
+ * Shows a zoom or a Fit as a short transition instead of a jump (#309). The surface changes its view
+ * as it always did, and the tween shows the way from what was on screen to it.
+ *
+ * Changing the view by other means – a pan, another document, a resize – ends the transition where
+ * it stands: the tween notices that the view is no longer the one it left and lets go.
+ */
+export class ViewTween {
+  private run: { from: ViewMap; to: ViewMap; start: number; ms: number; left: ViewMap } | null = null;
+  private readonly tick = () => this.step();
+
+  constructor(
+    private readonly view: TweenedView,
+    private readonly duration: () => number = () => generalSettings().zoomTransitionMs,
+    private readonly clock: TweenClock = tickerClock,
+  ) {}
+
+  /** Whether a transition is under way – the check scripts sample it. */
+  get moving(): boolean {
+    return this.run !== null;
+  }
+
+  /** Where the transition is heading, null while none is under way. */
+  get heading(): ViewMap | null {
+    return this.run ? { ...this.run.to } : null;
+  }
+
+  /**
+   * Carries out `change` and shows the way to the view it leaves. A change made during a transition
+   * starts from where that one was heading, so quick wheel steps add up to exactly as many steps,
+   * while the way to the new view starts from what is on screen.
+   */
+  change(change: () => void): void {
+    const shown = this.view.read();
+    const running = this.run;
+    if (running) this.view.write(running.to);
+    change();
+    const to = this.view.read();
+    const ms = this.duration();
+    if (ms <= 0 || near(shown, to)) {
+      this.stop();
+      if (running) this.view.draw();
+      return;
+    }
+    if (running && near(running.to, to)) {
+      this.view.write(shown);
+      return;
+    }
+    if (!running) this.clock.add(this.tick);
+    // Not drawn here: the next frame draws the first step of the way before it is rendered.
+    this.view.write(shown);
+    this.run = { from: shown, to, start: this.clock.now(), ms, left: this.view.read() };
+    requestRender();
+  }
+
+  /** Jumps to where the transition was heading and draws it – before a pan takes the view. */
+  finish(): void {
+    if (!this.run) return;
+    this.land();
+    this.view.draw();
+  }
+
+  /** Puts the view where the transition was heading, without drawing – before it is saved. */
+  land(): void {
+    const run = this.run;
+    if (!run) return;
+    this.stop();
+    this.view.write(run.to);
+  }
+
+  private step(): void {
+    const run = this.run;
+    if (!run) return;
+    if (!near(this.view.read(), run.left)) {
+      this.stop();
+      return;
+    }
+    const t = (this.clock.now() - run.start) / run.ms;
+    this.view.write(t >= 1 ? run.to : tweenView(run.from, run.to, easeOut(t)));
+    run.left = this.view.read();
+    if (t >= 1) this.stop();
+    this.view.draw();
+    requestRender();
+  }
+
+  private stop(): void {
+    if (!this.run) return;
+    this.run = null;
+    this.clock.remove(this.tick);
+  }
+}

src/editor/viewport/FreeViewport.ts

@@ -13,6 +13,7 @@ import {
   zoomStepBelow,
 } from '../../viewport/view';
 import { WorldViewport } from '../../viewport/WorldViewport';
+import { ViewTween } from '../viewTween';
 
 /** Breathing room left around the content by `fit()`, in screen pixels. */
 const FIT_MARGIN = 40;
@@ -29,6 +30,18 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
   /** Global position of the top-left corner at the last resize, for `resize()` to keep the world in place. */
   private screenOrigin = { x: 0, y: 0 };
   private readonly wheel = wheelSteps();
+  /** The view as a map from world to device pixels around the centre, which a transition keeps still. */
+  protected readonly tween = new ViewTween({
+    read: () => ({
+      scale: this.view.zoom,
+      x: -this.view.x * this.view.zoom,
+      y: -this.view.y * this.view.zoom,
+    }),
+    write: (map) => {
+      this.view = { x: -map.x / map.scale, y: -map.y / map.scale, zoom: map.scale };
+    },
+    draw: () => this.redraw(),
+  });
 
   protected constructor(state: EditorState) {
     super(state);
@@ -40,8 +53,10 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
    * placed and where the preview places its root, so content far off to one side would hide it.
    */
   fit(): void {
-    this.frameContent();
-    this.redraw();
+    this.tween.change(() => {
+      this.frameContent();
+      this.redraw();
+    });
   }
 
   /** Points the camera at everything drawn, without drawing. */
@@ -77,6 +92,7 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
    */
   override resize(w: number, h: number): void {
     if (this.w > 0 && this.h > 0 && (w !== this.w || h !== this.h)) {
+      this.tween.land();
       const before = this.screenOrigin;
       const after = this.getGlobalPosition();
       this.view.x += (after.x + w / 2 - (before.x + this.w / 2)) / this.worldScale();
@@ -88,6 +104,7 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
 
   /** Where the camera stands, to come back to (#171). */
   saveView(): unknown {
+    this.tween.land();
     return { ...this.view };
   }
 
@@ -116,6 +133,7 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
   }
 
   protected startPan(e: FederatedPointerEvent): void {
+    this.tween.finish();
     this.panning = { startX: e.global.x, startY: e.global.y, viewX: this.view.x, viewY: this.view.y };
     e.stopPropagation();
   }
@@ -139,14 +157,27 @@ export abstract class FreeViewport extends WorldViewport<EditorState> {
     e.preventDefault();
     const direction = this.wheel(e.deltaY);
     if (direction === 0) return;
-    const zoom = stepFreeZoom(this.view.zoom, direction);
-    if (zoom === this.view.zoom) return;
     const local = this.toLocal(e.global);
-    const before = screenToWorld(this.currentView(), local.x, local.y);
-    this.view.zoom = zoom;
-    const after = screenToWorld(this.currentView(), local.x, local.y);
-    this.view.x += before.x - after.x;
-    this.view.y += before.y - after.y;
-    this.redraw();
+    this.tween.change(() => {
+      const zoom = stepFreeZoom(this.view.zoom, direction);
+      if (zoom === this.view.zoom) return;
+      const before = screenToWorld(this.currentView(), local.x, local.y);
+      this.view.zoom = zoom;
+      const after = screenToWorld(this.currentView(), local.x, local.y);
+      this.view.x += before.x - after.x;
+      this.view.y += before.y - after.y;
+      this.redraw();
+    });
+  }
+
+  /** Centre, zoom and whether a transition is under way – what the check scripts sample (#309). */
+  viewState(): { x: number; y: number; zoom: number; moving: boolean } {
+    return { ...this.view, moving: this.tween.moving };
+  }
+
+  /** The world point shown at a global screen point, by the rule the wheel zooms with. */
+  worldAt(x: number, y: number): { x: number; y: number } {
+    const local = this.toLocal({ x, y });
+    return screenToWorld(this.currentView(), local.x, local.y);
   }
 }

src/editor/viewport/SceneViewport.ts

@@ -308,8 +308,10 @@ export class SceneViewport extends FreeViewport {
    * switching between the two tabs moves nothing. Without one, frames the whole content.
    */
   fitScene(): void {
-    this.frameScene();
-    this.redraw();
+    this.tween.change(() => {
+      this.frameScene();
+      this.redraw();
+    });
   }
 
   private frameScene(): void {