Preserve iOS chat scroll momentum

Packages/iOS/CmuxAgentChatUI/Sources/CmuxAgentChatUI/Transcript/ChatTranscriptTableView.swift

@@ -145,11 +145,10 @@ struct ChatTranscriptTableView: UIViewRepresentable {
             defer { isApplyingDataUpdate = false }
             tableView.reloadData()
             tableView.layoutIfNeeded()
-
-            if shouldScrollToBottom || wasAtBottom {
+            if shouldScrollToBottom || (wasAtBottom && !tableView.isUserScrollMomentumActive) {
                 pendingContentUpdateAnchor = nil
                 scrollToBottom(in: tableView, animated: false)
-            } else if let anchor {
+            } else if let anchor, !tableView.isUserScrollMomentumActive {
                 restore(anchor, in: tableView)
                 pendingContentUpdateAnchor = anchor
             }
@@ -214,11 +213,12 @@ struct ChatTranscriptTableView: UIViewRepresentable {
             isHandlingLayout = true
             defer { isHandlingLayout = false }
 
-            if tableView.isViewportInsetsExternallyDriven {
+            if tableView.isUserScrollMomentumActive {
+                pendingContentUpdateAnchor = nil
                 updateBottomState(from: tableView)
                 return
             }
-            if isApplyingDataUpdate {
+            if tableView.isViewportInsetsExternallyDriven || isApplyingDataUpdate {
                 updateBottomState(from: tableView)
                 return
             }

Packages/iOS/CmuxAgentChatUI/Sources/CmuxAgentChatUI/Transcript/ChatTranscriptUITableView.swift

@@ -126,6 +126,10 @@ final class ChatTranscriptUITableView: UITableView {
         recordCurrentViewport()
     }
 
+    var isUserScrollMomentumActive: Bool {
+        return isTracking || isDragging || isDecelerating
+    }
+
     func applyTranscriptViewportInsets(
         topChromeInset: CGFloat,
         adjustedBottomInset: CGFloat,
@@ -162,6 +166,12 @@ final class ChatTranscriptUITableView: UITableView {
         indicatorInsets.top = resolvedTopInset
         indicatorInsets.bottom = resolvedOverlayBottomInset
         verticalScrollIndicatorInsets = indicatorInsets
+        if isUserScrollMomentumActive {
+            // Preserve UIKit's live inset compensation while drag/deceleration owns the offset.
+            recordCurrentViewport()
+            isViewportInsetsExternallyDriven = false
+            return
+        }
         if wasPinnedToTop {
             // Keep the transcript pinned to the top chrome reservation — the
             // symmetric counterpart to `wasAtBottom`. Without this, a
@@ -236,7 +246,7 @@ final class ChatTranscriptUITableView: UITableView {
         let presentationGap = composerPresentationMinY - presentationFrameMaxY
         recordKeyboardAnimationGap(presentationGap)
         return String(
-            format: "frameMinY=%.2f;frameMaxY=%.2f;frameHeight=%.2f;presentationFrameMaxY=%.2f;boundsHeight=%.2f;offsetY=%.2f;adjustedTopInset=%.2f;adjustedBottomInset=%.2f;visibleTopY=%.2f;visibleBottomY=%.2f;contentHeight=%.2f;distanceFromBottom=%.2f;keyboardEvents=%d;keyboardOverlap=%.2f;keyboardTargetOverlap=%.2f;keyboardGuideOverlap=%.2f;keyboardBottomConstraint=%.2f;composerMinY=%.2f;composerPresentationMinY=%.2f;presentationGap=%.2f;topChromeOverlayInset=%.2f;composerOverlayBottomInset=%.2f;keyboardAnimationActive=%d;keyboardAnimationProgress=%.2f;keyboardTransitionDuration=%.3f;maxAnimationPresentationGap=%.2f;keyboardAnimationSamples=%d;topEdgeEffectSoft=%d;bottomEdgeEffectSoft=%d;topContentScrollViewRegistered=%d",
+            format: "frameMinY=%.2f;frameMaxY=%.2f;frameHeight=%.2f;presentationFrameMaxY=%.2f;boundsHeight=%.2f;offsetY=%.2f;adjustedTopInset=%.2f;adjustedBottomInset=%.2f;visibleTopY=%.2f;visibleBottomY=%.2f;contentHeight=%.2f;distanceFromBottom=%.2f;keyboardEvents=%d;keyboardOverlap=%.2f;keyboardTargetOverlap=%.2f;keyboardGuideOverlap=%.2f;keyboardBottomConstraint=%.2f;composerMinY=%.2f;composerPresentationMinY=%.2f;presentationGap=%.2f;topChromeOverlayInset=%.2f;composerOverlayBottomInset=%.2f;keyboardAnimationActive=%d;keyboardAnimationProgress=%.2f;keyboardTransitionDuration=%.3f;maxAnimationPresentationGap=%.2f;keyboardAnimationSamples=%d;topEdgeEffectSoft=%d;bottomEdgeEffectSoft=%d;topContentScrollViewRegistered=%d;scrollTracking=%d;scrollDragging=%d;scrollDecelerating=%d",
             locale: Locale(identifier: "en_US_POSIX"),
             frameInWindow.minY,
             frameInWindow.maxY,
@@ -267,7 +277,10 @@ final class ChatTranscriptUITableView: UITableView {
             keyboardDebugAnimationSampleCount,
             debugTopEdgeEffectSoft ? 1 : 0,
             debugBottomEdgeEffectSoft ? 1 : 0,
-            debugTopContentScrollViewRegistered ? 1 : 0
+            debugTopContentScrollViewRegistered ? 1 : 0,
+            isTracking ? 1 : 0,
+            isDragging ? 1 : 0,
+            isDecelerating ? 1 : 0
         )
     }
 

ios/cmuxUITests/cmuxUITests.swift

@@ -1203,6 +1203,49 @@ final class cmuxUITests: XCTestCase {
         }
     }
 
+    @MainActor
+    func testAgentChatTranscriptFastSwipeEvidence() throws {
+        let app = launchAgentChatInlinePreviewApp(environment: [
+            "CMUX_UITEST_CHAT_INITIAL_SCROLL": "middle",
+        ])
+        let table = app.tables["ChatTranscriptTableView"]
+        XCTAssertTrue(table.waitForExistence(timeout: 8))
+
+        let before = try waitForTranscriptMetrics(table, timeout: 8) {
+            $0.frameHeight > 240
+                && $0.contentHeight > $0.boundsHeight * 1.6
+                && $0.offsetY > 80
+                && $0.distanceFromBottom > 220
+        }
+        captureKeyboardEvidenceFrame(
+            prefix: "scroll-deceleration-before",
+            index: 0,
+            startedAt: Date(),
+            metrics: before
+        )
+
+        table.swipeUp(velocity: .fast)
+        let afterSwipe = try waitForTranscriptMetrics(table, timeout: 1.5) {
+            $0.offsetY > before.offsetY + 40
+        }
+        captureKeyboardEvidenceFrame(
+            prefix: "scroll-deceleration-after",
+            index: 0,
+            startedAt: Date(),
+            metrics: afterSwipe
+        )
+        XCTAssertGreaterThan(
+            afterSwipe.offsetY,
+            before.offsetY + 40,
+            "A fast transcript swipe should move through the chat history instead of being swallowed by parent gesture handling. before=\(before) after=\(afterSwipe)"
+        )
+        XCTAssertGreaterThan(
+            afterSwipe.distanceFromBottom,
+            80,
+            "A single fast swipe from the middle fixture must not snap to the live bottom. before=\(before) after=\(afterSwipe)"
+        )
+    }
+
     @MainActor
     func testAgentChatExpansionControlsPreserveTranscriptScrollPosition() throws {
         let app = launchAgentChatInlinePreviewApp()
@@ -2202,9 +2245,12 @@ final class cmuxUITests: XCTestCase {
         let topEdgeEffectSoft: Bool
         let bottomEdgeEffectSoft: Bool
         let topContentScrollViewRegistered: Bool
+        let scrollTracking: Bool
+        let scrollDragging: Bool
+        let scrollDecelerating: Bool
 
         var description: String {
-            "frameMinY=\(frameMinY), frameMaxY=\(frameMaxY), frameHeight=\(frameHeight), presentationFrameMaxY=\(presentationFrameMaxY), boundsHeight=\(boundsHeight), offsetY=\(offsetY), adjustedTopInset=\(adjustedTopInset), adjustedBottomInset=\(adjustedBottomInset), visibleTopY=\(visibleTopY), visibleBottomY=\(visibleBottomY), contentHeight=\(contentHeight), distanceFromBottom=\(distanceFromBottom), keyboardEvents=\(keyboardEvents), keyboardOverlap=\(keyboardOverlap), keyboardTargetOverlap=\(keyboardTargetOverlap), composerMinY=\(composerMinY), composerPresentationMinY=\(composerPresentationMinY), presentationGap=\(presentationGap), topChromeOverlayInset=\(topChromeOverlayInset), composerOverlayBottomInset=\(composerOverlayBottomInset), keyboardAnimationActive=\(keyboardAnimationActive), keyboardAnimationProgress=\(keyboardAnimationProgress), keyboardTransitionDuration=\(keyboardTransitionDuration), maxAnimationPresentationGap=\(maxAnimationPresentationGap), keyboardAnimationSamples=\(keyboardAnimationSamples), topEdgeEffectSoft=\(topEdgeEffectSoft), bottomEdgeEffectSoft=\(bottomEdgeEffectSoft), topContentScrollViewRegistered=\(topContentScrollViewRegistered)"
+            "frameMinY=\(frameMinY), frameMaxY=\(frameMaxY), frameHeight=\(frameHeight), presentationFrameMaxY=\(presentationFrameMaxY), boundsHeight=\(boundsHeight), offsetY=\(offsetY), adjustedTopInset=\(adjustedTopInset), adjustedBottomInset=\(adjustedBottomInset), visibleTopY=\(visibleTopY), visibleBottomY=\(visibleBottomY), contentHeight=\(contentHeight), distanceFromBottom=\(distanceFromBottom), keyboardEvents=\(keyboardEvents), keyboardOverlap=\(keyboardOverlap), keyboardTargetOverlap=\(keyboardTargetOverlap), composerMinY=\(composerMinY), composerPresentationMinY=\(composerPresentationMinY), presentationGap=\(presentationGap), topChromeOverlayInset=\(topChromeOverlayInset), composerOverlayBottomInset=\(composerOverlayBottomInset), keyboardAnimationActive=\(keyboardAnimationActive), keyboardAnimationProgress=\(keyboardAnimationProgress), keyboardTransitionDuration=\(keyboardTransitionDuration), maxAnimationPresentationGap=\(maxAnimationPresentationGap), keyboardAnimationSamples=\(keyboardAnimationSamples), topEdgeEffectSoft=\(topEdgeEffectSoft), bottomEdgeEffectSoft=\(bottomEdgeEffectSoft), topContentScrollViewRegistered=\(topContentScrollViewRegistered), scrollTracking=\(scrollTracking), scrollDragging=\(scrollDragging), scrollDecelerating=\(scrollDecelerating)"
         }
 
         var effectiveFrameMaxY: CGFloat {
@@ -2259,6 +2305,9 @@ final class cmuxUITests: XCTestCase {
             self.topEdgeEffectSoft = (values["topEdgeEffectSoft"] ?? 0) >= 0.5
             self.bottomEdgeEffectSoft = (values["bottomEdgeEffectSoft"] ?? 0) >= 0.5
             self.topContentScrollViewRegistered = (values["topContentScrollViewRegistered"] ?? 0) >= 0.5
+            self.scrollTracking = (values["scrollTracking"] ?? 0) >= 0.5
+            self.scrollDragging = (values["scrollDragging"] ?? 0) >= 0.5
+            self.scrollDecelerating = (values["scrollDecelerating"] ?? 0) >= 0.5
         }
     }