Fix stale browser pane content after drag splits

Sources/BrowserWindowPortal.swift

@@ -2786,9 +2786,11 @@ final class WindowBrowserPortal: NSObject {
             }
             return
         }
+        let previousTransientRecoveryReason = entry.transientRecoveryReason
         func hideContainerView(reason: String) {
             containerView.setPaneTopChromeHeight(0)
             containerView.setSearchOverlay(nil)
+            containerView.setPaneDropContext(nil)
             containerView.setDropZoneOverlay(zone: nil)
             if !containerView.isHidden, webView.superview === containerView {
                 webView.browserPortalNotifyHidden(reason: reason)
@@ -2819,6 +2821,7 @@ final class WindowBrowserPortal: NSObject {
                 "reason=\(reason) frame=\(browserPortalDebugFrame(containerView.frame))"
             )
 #endif
+            containerView.setPaneDropContext(nil)
             containerView.setDropZoneOverlay(zone: nil)
             return true
         }
@@ -2962,6 +2965,8 @@ final class WindowBrowserPortal: NSObject {
                         "reason=hostBoundsNotReady frame=\(browserPortalDebugFrame(containerView.frame))"
                     )
 #endif
+                    containerView.setPaneDropContext(nil)
+                    containerView.setDropZoneOverlay(zone: nil)
                     return
                 }
             } else {
@@ -3024,6 +3029,9 @@ final class WindowBrowserPortal: NSObject {
             shouldHide &&
             entry.visibleInUI &&
             !containerView.isHidden
+        let recoveredFromTransientGeometry =
+            previousTransientRecoveryReason != nil &&
+            transientRecoveryReason == nil
 #if DEBUG
         let frameWasClamped = hasFiniteFrame && !Self.rectApproximatelyEqual(frameInHost, targetFrame)
         if frameWasClamped {
@@ -3152,10 +3160,16 @@ final class WindowBrowserPortal: NSObject {
         }
         containerView.setPaneTopChromeHeight(shouldHide ? 0 : entry.paneTopChromeHeight)
         containerView.setSearchOverlay(shouldHide ? nil : entry.searchOverlay)
+        containerView.setPaneDropContext(containerView.isHidden ? nil : entry.paneDropContext)
         containerView.setDropZoneOverlay(zone: containerView.isHidden ? nil : entry.dropZone)
         if revealedForDisplay {
             refreshReasons.append("reveal")
         }
+        if recoveredFromTransientGeometry {
+            // Drag/reparent churn can recover to the same visible frame we preserved.
+            // Force a redraw so WebKit doesn't keep stale tiles until a later resize/focus.
+            refreshReasons.append("transientRecovery")
+        }
         if forcePresentationRefresh {
             refreshReasons.append("anchor")
         }

Sources/Workspace.swift

@@ -4896,6 +4896,21 @@ extension Workspace: BonsplitDelegate {
             "originalKinds=[\(paneKindSummary(originalPane))] newKinds=[\(paneKindSummary(newPane))]"
         )
 #endif
+        let rearmBrowserPortalHostReplacement: (PaneID, String) -> Void = { paneId, reason in
+            for tab in controller.tabs(inPane: paneId) {
+                guard let panelId = self.panelIdFromSurfaceId(tab.id),
+                      let browserPanel = self.browserPanel(for: panelId) else {
+                    continue
+                }
+                browserPanel.preparePortalHostReplacementForNextDistinctClaim(
+                    inPane: paneId,
+                    reason: reason
+                )
+            }
+        }
+        rearmBrowserPortalHostReplacement(originalPane, "workspace.didSplit.original")
+        rearmBrowserPortalHostReplacement(newPane, "workspace.didSplit.new")
+
         // Only auto-create a terminal if the split came from bonsplit UI.
         // Programmatic splits via newTerminalSplit() set isProgrammaticSplit and handle their own panels.
         guard !isProgrammaticSplit else {