dashboard: pan canvas via middle-drag and horizontal scroll over browser/chat

- Middle-mouse drag inside a browser webview is forwarded as canvas pan
  (previously eaten silently by the guest compositor).
- Horizontal-dominant wheel inside a browser webview pans the canvas
  when no ancestor in the guest page can absorb horizontal scroll;
  otherwise the page handles it.
- Horizontal-dominant wheel over a chat panel pans the canvas (chat has
  no horizontal scroller). Vertical scroll is unchanged.

Plain vertical scroll and ctrl/meta+wheel zoom over a browser stay with
chromium's defaults.

electron/webview-preload.js

@@ -137,39 +137,59 @@ try {
   });
 
   // ---------------------------------------------------------------------------
-  // Canvas zoom passthrough (ctrl/meta + wheel)
+  // Horizontal scroll passthrough to canvas pan
   //
-  // A <webview> is an out-of-process Chromium guest; wheel events that
-  // originate inside it never bubble to the embedding renderer. Without
-  // intercepting here, ctrl+wheel over a browser card just zooms the
-  // embedded page (Chromium's default) and the dashboard canvas never
-  // sees the gesture — issue #27.
-  //
-  // Capture-phase + passive:false so we run before the page's own listeners
-  // and can preventDefault to suppress the in-page page-zoom. We then
-  // forward the gesture (deltaY + guest-local cursor coords) to the host
-  // via sendToHost; BrowserCard's ipc-message handler turns it back into a
-  // synthetic WheelEvent dispatched from the webview element, which bubbles
-  // naturally to useCanvasControls' wheel listener.
+  // <webview> is an out-of-process guest; wheel events inside it never bubble
+  // to the embedding renderer. Vertical scroll and ctrl/meta+wheel zoom stay
+  // with the page (chromium default). A horizontal-dominant gesture, however,
+  // should pan the dashboard canvas if the guest page has nothing horizontal
+  // to scroll, to match the behavior over chat panels (which never have a
+  // horizontal scroller and always pan the canvas).
+  const pageCanScrollX = (node, dx) => {
+    let t = node;
+    while (t) {
+      const sw = t.scrollWidth || 0;
+      const cw = t.clientWidth || 0;
+      if (sw > cw) {
+        let style;
+        try { style = getComputedStyle(t); } catch (_) {}
+        const ox = style ? style.overflowX : 'visible';
+        if (ox === 'auto' || ox === 'scroll') {
+          const atRight = t.scrollLeft + cw >= sw - 1;
+          const atLeft = t.scrollLeft <= 1;
+          const atBoundary = (dx > 0 && atRight) || (dx < 0 && atLeft);
+          if (!atBoundary) return true;
+        }
+      }
+      t = t.parentElement;
+    }
+    const docEl = document.scrollingElement || document.documentElement;
+    if (docEl && docEl.scrollWidth > docEl.clientWidth) {
+      const atRight = docEl.scrollLeft + docEl.clientWidth >= docEl.scrollWidth - 1;
+      const atLeft = docEl.scrollLeft <= 1;
+      const atBoundary = (dx > 0 && atRight) || (dx < 0 && atLeft);
+      if (!atBoundary) return true;
+    }
+    return false;
+  };
+
   const onWheelCapture = (e) => {
-    if (!(e.ctrlKey || e.metaKey)) return;
+    // Pinch / ctrl+wheel stays with the page (chromium's in-page zoom).
+    if (e.ctrlKey || e.metaKey) return;
+    // Vertical-dominant scroll stays with the page.
+    if (Math.abs(e.deltaX) <= Math.abs(e.deltaY)) return;
+    // Horizontal-dominant: defer to the page if anything inside can absorb
+    // it; otherwise forward to the host as a canvas pan.
+    if (pageCanScrollX(e.target, e.deltaX)) return;
     e.preventDefault();
     e.stopPropagation();
     try {
-      console.warn('[openswarm:webview-preload] ctrl+wheel intercept → sendToHost', {
-        deltaY: e.deltaY,
-        clientX: e.clientX,
-        clientY: e.clientY,
-      });
-      ipcRenderer.sendToHost('canvas-wheel-zoom', {
+      ipcRenderer.sendToHost('canvas-wheel-pan', {
+        deltaX: e.deltaX,
         deltaY: e.deltaY,
         deltaMode: e.deltaMode,
-        clientX: e.clientX,
-        clientY: e.clientY,
       });
-    } catch (err) {
-      console.warn('[openswarm:webview-preload] sendToHost failed', err);
-    }
+    } catch (_) {}
   };
   // Listen on both window and document in capture phase so we run before any
   // page-level handler that might swallow the event. passive:false is required
@@ -177,6 +197,48 @@ try {
   window.addEventListener('wheel', onWheelCapture, { capture: true, passive: false });
   document.addEventListener('wheel', onWheelCapture, { capture: true, passive: false });
 
+  // ---------------------------------------------------------------------------
+  // Middle-mouse-button drag → canvas pan
+  //
+  // Empty canvas and agent cards already get middle-button pan because the
+  // event bubbles to the dashboard's mousedown handler. <webview> is a
+  // separate compositor layer that eats mouse events, so middle-drag over a
+  // browser silently did nothing. Intercept here and forward the per-event
+  // movement as a pan delta through the existing canvas-wheel-pan channel
+  // (negated, since drag pans panX += dx while wheel pans panX -= dx).
+  // Always pans regardless of capture state — middle-drag is unambiguously
+  // a canvas gesture.
+  let middleDragging = false;
+  const onMouseDownMiddle = (e) => {
+    if (e.button !== 1) return;
+    e.preventDefault();
+    e.stopPropagation();
+    middleDragging = true;
+  };
+  const onMouseMoveMiddle = (e) => {
+    if (!middleDragging) return;
+    e.preventDefault();
+    e.stopPropagation();
+    const dx = e.movementX || 0;
+    const dy = e.movementY || 0;
+    if (dx === 0 && dy === 0) return;
+    try {
+      ipcRenderer.sendToHost('canvas-wheel-pan', { deltaX: -dx, deltaY: -dy, deltaMode: 0 });
+    } catch (_) {}
+  };
+  const onMouseUpMiddle = (e) => {
+    if (e.button !== 1) return;
+    middleDragging = false;
+  };
+  // Chromium starts auxiliary-scroll on middle-click; auxclick prevents that.
+  const onAuxClickSuppress = (e) => {
+    if (e.button === 1) { e.preventDefault(); e.stopPropagation(); }
+  };
+  window.addEventListener('mousedown', onMouseDownMiddle, { capture: true });
+  window.addEventListener('mousemove', onMouseMoveMiddle, { capture: true });
+  window.addEventListener('mouseup', onMouseUpMiddle, { capture: true });
+  window.addEventListener('auxclick', onAuxClickSuppress, { capture: true });
+
   // ---------------------------------------------------------------------------
   // Double-click to fit the browser card (parity with agent-chat dblclick).
   //

frontend/src/app/pages/AgentChat/AgentChat.tsx

@@ -664,6 +664,9 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
       // Without this early-out the unconditional stopPropagation below kills
       // ctrl+wheel and the canvas listener never fires.
       if (e.ctrlKey || e.metaKey) return;
+      // Horizontal-dominant gestures must also reach the canvas so a sideways
+      // swipe pans the dashboard (chat has no horizontal scroll to absorb).
+      if (Math.abs(e.deltaX) > Math.abs(e.deltaY)) return;
       const atTop = el.scrollTop <= 0;
       const atBottom = el.scrollTop + el.clientHeight >= el.scrollHeight - 1;
       const scrollingDown = e.deltaY > 0;

frontend/src/app/pages/Dashboard/cards/BrowserCard.tsx

@@ -303,7 +303,7 @@ const BrowserCard: React.FC<Props> = ({
           // (the historical Windows mount segfault). Clear the crash-safety marker.
           if (isWindows) markWindowsWebviewSurvived();
           wv.loadURL(targetUrl).catch(() => {});
-          // Lock guest zoom at 1.0 so ctrl+wheel never triggers Chromium's in-page zoom; canvas zoom takes over (issue #27).
+          // Disable in-guest pinch zoom; page zoom (cmd+= / cmd+-) still works via chromium.
           try {
             (wv as any).setVisualZoomLevelLimits?.(1, 1);
             (wv as any).setZoomFactor?.(1);
@@ -328,19 +328,16 @@ const BrowserCard: React.FC<Props> = ({
           setPasskeyDialogOpen(true);
         } else if (e?.channel === 'browser-dblclick') {
           onDoubleClickRef.current?.(browserId, 'browser');
-        } else if (e?.channel === 'canvas-wheel-zoom') {
-          // Convert guest coords to doc coords and dispatch a CustomEvent; synthetic WheelEvent bubble was unreliable through GuestView.
+        } else if (e?.channel === 'canvas-wheel-pan') {
+          // Plain wheel inside an unselected webview never bubbles out; the
+          // preload forwards it here so the dashboard canvas can pan.
           const payload = e.args?.[0] || {};
-          const wvRect = wv.getBoundingClientRect();
-          const docX = wvRect.left + (payload.clientX ?? 0);
-          const docY = wvRect.top + (payload.clientY ?? 0);
           window.dispatchEvent(
-            new CustomEvent('openswarm:canvas-wheel-zoom', {
+            new CustomEvent('openswarm:canvas-wheel-pan', {
               detail: {
+                deltaX: payload.deltaX ?? 0,
                 deltaY: payload.deltaY ?? 0,
                 deltaMode: payload.deltaMode ?? 0,
-                clientX: docX,
-                clientY: docY,
               },
             }),
           );

frontend/src/app/pages/Dashboard/hooks/interaction/useCanvasControls.ts

@@ -261,6 +261,15 @@ export function useCanvasControls(zoomSensitivity: number = 50, contentBounds?:
           // Re-read scrollHeight/clientHeight; cached decision is structural, scroll position is dynamic.
           const canScrollY = target.scrollHeight > target.clientHeight;
           const canScrollX = target.scrollWidth > target.clientWidth;
+
+          // Horizontal-dominant gestures over a container that only scrolls
+          // vertically (e.g., chat) should pan the canvas instead of being
+          // silently absorbed by the child's no-op horizontal handling.
+          if (Math.abs(dx) > Math.abs(dy) && !canScrollX) {
+            target = target.parentElement;
+            continue;
+          }
+
           const atYBoundary = !canScrollY ||
             (dy > 0 && target.scrollTop + target.clientHeight >= target.scrollHeight - 1) ||
             (dy < 0 && target.scrollTop <= 1);
@@ -302,31 +311,26 @@ export function useCanvasControls(zoomSensitivity: number = 50, contentBounds?:
 
     el.addEventListener('wheel', onWheel, { passive: false });
 
-    // ctrl/meta+wheel events that originate inside an Electron <webview>
-    // never bubble out of the guest into the host DOM, so the wheel
-    // listener above can't see them. BrowserCard's preload-bridge
-    // forwards those gestures via this CustomEvent (issue #27); we run
-    // the same zoom-around-cursor math the wheel handler uses.
-    const onForwardedZoom = (e: Event) => {
+    // Plain wheel inside a webview can't bubble out either; the preload
+    // forwards horizontal-dominant scrolls as a pan when the guest page
+    // has nothing to scroll horizontally, plus middle-mouse drag deltas.
+    const onForwardedPan = (e: Event) => {
       const detail = (e as CustomEvent).detail || {};
-      const dy = detail.deltaMode === 1 ? detail.deltaY * 40 : detail.deltaY;
-      const rect = el.getBoundingClientRect();
+      const dy = detail.deltaMode === 1 ? (detail.deltaY ?? 0) * 40 : (detail.deltaY ?? 0);
+      const dx = detail.deltaMode === 1 ? (detail.deltaX ?? 0) * 40 : (detail.deltaX ?? 0);
       if (inertiaFrameRef.current) {
         cancelAnimationFrame(inertiaFrameRef.current);
         inertiaFrameRef.current = null;
       }
-      pendingZoomDy += dy;
-      pendingZoomCenter = {
-        cx: (detail.clientX ?? 0) - rect.left,
-        cy: (detail.clientY ?? 0) - rect.top,
-      };
+      pendingPanDx += dx;
+      pendingPanDy += dy;
       scheduleWheelFlush();
     };
-    window.addEventListener('openswarm:canvas-wheel-zoom', onForwardedZoom);
+    window.addEventListener('openswarm:canvas-wheel-pan', onForwardedPan);
 
     return () => {
       el.removeEventListener('wheel', onWheel);
-      window.removeEventListener('openswarm:canvas-wheel-zoom', onForwardedZoom);
+      window.removeEventListener('openswarm:canvas-wheel-pan', onForwardedPan);
       if (wheelRafId != null) cancelAnimationFrame(wheelRafId);
       if (wheelIdleTimer != null) clearTimeout(wheelIdleTimer);
       // Don't leave the flag stuck on if the canvas unmounts mid-gesture.

frontend/src/app/pages/Dashboard/hooks/interaction/useOverlayScrollPassthrough.ts

@@ -22,6 +22,8 @@ export function useOverlayScrollPassthrough(active: boolean) {
         dy *= 20;
       }
 
+      const horizontalDominant = Math.abs(dx) > Math.abs(dy);
+
       let node = underneath as HTMLElement | null;
       while (node) {
         if (node.tagName === 'WEBVIEW') {
@@ -52,6 +54,14 @@ export function useOverlayScrollPassthrough(active: boolean) {
           node.scrollWidth > node.clientWidth &&
           (cs.overflowX === 'auto' || cs.overflowX === 'scroll');
 
+        // Horizontal-dominant gesture over a vertically-only scrollable
+        // container: don't absorb it (scrollBy with dx would be a no-op).
+        // Let it bubble to the canvas wheel handler so the canvas pans.
+        if (horizontalDominant && !canScrollX) {
+          node = node.parentElement;
+          continue;
+        }
+
         if (canScrollY || canScrollX) {
           e.stopPropagation();
           e.preventDefault();