fix: preserve viewport mode during trackpad scroll

client/src/app/AppShell.tsx

@@ -78,6 +78,7 @@ import type {
 } from "../features/viewport/types";
 import { useViewportLayout } from "../features/viewport/useViewportLayout";
 import { NewSimulatorModal } from "../features/simulators/NewSimulatorModal";
+import { nextViewportWheelPanState } from "../features/viewport/viewportWheel";
 import {
   buildShellRotationTransform,
   clampPan,
@@ -2097,20 +2098,22 @@ export function AppShell({
       return;
     }
 
-    setViewMode("manual");
-    setPan((currentPan) =>
-      clampPan(
-        {
-          x: currentPan.x - deltaX,
-          y: currentPan.y + autoViewportOffsetY - deltaY,
-        },
-        effectiveZoom,
-        canvasSize,
-        effectiveDeviceNaturalSize,
-        viewportChromeProfile,
-        rotationQuarterTurns,
-        zoomDockReservedHeight,
-      ),
+    setPan(
+      (currentPan) =>
+        nextViewportWheelPanState({
+          canvasSize,
+          chromeProfile: viewportChromeProfile,
+          deltaX,
+          deltaY,
+          deviceNaturalSize: effectiveDeviceNaturalSize,
+          effectiveZoom,
+          fitScale,
+          pan: currentPan,
+          reservedBottomInset: zoomDockReservedHeight,
+          rotationQuarterTurns,
+          viewMode,
+          zoom,
+        }).pan,
     );
   }
 

client/src/features/viewport/viewportWheel.test.ts

@@ -0,0 +1,76 @@
+import { describe, expect, it } from "vitest";
+
+import { nextViewportWheelPanState } from "./viewportWheel";
+
+describe("nextViewportWheelPanState", () => {
+  it("preserves fit mode and zoom when a fitted viewport cannot pan", () => {
+    const pan = { x: 0, y: 0 };
+
+    expect(
+      nextViewportWheelPanState({
+        canvasSize: { width: 400, height: 800 },
+        chromeProfile: null,
+        deltaX: 0,
+        deltaY: 120,
+        deviceNaturalSize: { width: 300, height: 600 },
+        effectiveZoom: 1,
+        fitScale: 1,
+        pan,
+        reservedBottomInset: 96,
+        rotationQuarterTurns: 0,
+        viewMode: "fit",
+        zoom: null,
+      }),
+    ).toEqual({
+      pan,
+      viewMode: "fit",
+      zoom: null,
+    });
+  });
+
+  it("preserves center mode and zoom while panning", () => {
+    expect(
+      nextViewportWheelPanState({
+        canvasSize: { width: 300, height: 300 },
+        chromeProfile: null,
+        deltaX: 10,
+        deltaY: 20,
+        deviceNaturalSize: { width: 600, height: 600 },
+        effectiveZoom: 1,
+        fitScale: 0.5,
+        pan: { x: 0, y: 0 },
+        reservedBottomInset: 96,
+        rotationQuarterTurns: 0,
+        viewMode: "center",
+        zoom: null,
+      }),
+    ).toEqual({
+      pan: { x: -10, y: -20 },
+      viewMode: "center",
+      zoom: null,
+    });
+  });
+
+  it("keeps manual zoom during plain wheel panning", () => {
+    expect(
+      nextViewportWheelPanState({
+        canvasSize: { width: 300, height: 300 },
+        chromeProfile: null,
+        deltaX: 10,
+        deltaY: 20,
+        deviceNaturalSize: { width: 600, height: 600 },
+        effectiveZoom: 1.35,
+        fitScale: 0.5,
+        pan: { x: 0, y: 0 },
+        reservedBottomInset: 96,
+        rotationQuarterTurns: 0,
+        viewMode: "manual",
+        zoom: 1.35,
+      }),
+    ).toEqual({
+      pan: { x: -10, y: -20 },
+      viewMode: "manual",
+      zoom: 1.35,
+    });
+  });
+});

client/src/features/viewport/viewportWheel.ts

@@ -0,0 +1,63 @@
+import type { ChromeProfile } from "../../api/types";
+import type { Point, Size, ViewMode } from "./types";
+import { clampPan } from "./viewportMath";
+
+const PAN_EPSILON = 0.001;
+
+interface ViewportWheelState {
+  viewMode: ViewMode;
+  zoom: number | null;
+}
+
+interface ViewportWheelPanOptions {
+  canvasSize: Size | null;
+  chromeProfile: ChromeProfile | null;
+  deltaX: number;
+  deltaY: number;
+  deviceNaturalSize: Size | null;
+  effectiveZoom: number;
+  fitScale: number;
+  pan: Point;
+  reservedBottomInset: number;
+  rotationQuarterTurns: number;
+  viewMode: ViewMode;
+  zoom: number | null;
+}
+
+export function nextViewportWheelPanState({
+  canvasSize,
+  chromeProfile,
+  deltaX,
+  deltaY,
+  deviceNaturalSize,
+  effectiveZoom,
+  fitScale,
+  pan,
+  reservedBottomInset,
+  rotationQuarterTurns,
+  viewMode,
+  zoom,
+}: ViewportWheelPanOptions): ViewportWheelState & { pan: Point } {
+  if (effectiveZoom <= fitScale + PAN_EPSILON) {
+    return { pan, viewMode, zoom };
+  }
+
+  const nextPan = clampPan(
+    {
+      x: pan.x - deltaX,
+      y: pan.y - deltaY,
+    },
+    effectiveZoom,
+    canvasSize,
+    deviceNaturalSize,
+    chromeProfile,
+    rotationQuarterTurns,
+    viewMode === "manual" ? reservedBottomInset : 0,
+  );
+
+  return {
+    pan: nextPan,
+    viewMode,
+    zoom,
+  };
+}