Divider drags are sessions; imposed splits stop fighting foreign resizes

Sources/Bonsplit/Internal/Controllers/SplitViewController.swift

@@ -62,6 +62,26 @@ final class SplitViewController {
     /// Callback for geometry changes
     var onGeometryChange: (() -> Void)?
 
+    /// Live divider drag sessions across every split in this tree (0 or 1 in
+    /// practice — AppKit tracks one divider at a time). Sessions bracket the
+    /// divider's mouse-tracking lifecycle, so external sizing can consult
+    /// this before writing geometry: mid-drag the user owns the divider.
+    @ObservationIgnored private(set) var activeDividerDragSessions = 0
+    @ObservationIgnored var onDividerDragSessionChange: ((Bool) -> Void)?
+
+    func noteDividerDragSession(_ active: Bool) {
+        // Notify only when the count crosses zero: with overlapping sessions
+        // (a host-bracketed custom drag alongside the built-in tracking),
+        // ending one must not announce "drag over" while the other still
+        // owns the divider — a host would resume imposing under the pointer.
+        let wasActive = activeDividerDragSessions > 0
+        activeDividerDragSessions = max(0, activeDividerDragSessions + (active ? 1 : -1))
+        let isActive = activeDividerDragSessions > 0
+        if wasActive != isActive {
+            onDividerDragSessionChange?(isActive)
+        }
+    }
+
     init(rootNode: SplitNode? = nil) {
         if let rootNode {
             self.rootNode = rootNode

Sources/Bonsplit/Internal/Views/SplitContainerView.swift

@@ -65,6 +65,40 @@ private class ThemedSplitView: NSSplitView {
     // See `NonDraggableHostingView` in SplitNodeView.swift for the rest of
     // the chain.
     override var mouseDownCanMoveWindow: Bool { false }
+
+    /// Brackets a divider drag as a session: fires `true` when a mouseDown
+    /// lands on the divider's effective hit rect, `false` when AppKit's
+    /// divider tracking loop returns at mouseUp. Deterministic — taken from
+    /// the mouse lifecycle itself, never inferred from which event happens
+    /// to be current when a resize callback fires (that inference misses a
+    /// drag-pause-release, where no resize coincides with the mouseUp).
+    var onDividerDragSession: ((Bool) -> Void)?
+
+    override func mouseDown(with event: NSEvent) {
+        // Any mouseDown that reaches the split view itself is a divider
+        // interaction: arranged subviews cover all pane content, so content
+        // clicks never route here, and reconstructing AppKit's exact
+        // effective divider rect (drawn rect grown by the delegate's
+        // expansion, UNIONED with AppKit's own proposal) cannot be done
+        // faithfully from here — a rect test narrower than AppKit's would
+        // let AppKit track a drag with no session, and sizing would impose
+        // under the pointer. A session around a click that AppKit does not
+        // turn into a drag is harmless: begin and end fire back to back and
+        // the host's drag-end sync finds nothing changed.
+        let inDivider = arrangedSubviews.count >= 2
+#if DEBUG
+        if inDivider {
+            let location = convert(event.locationInWindow, from: nil)
+            dlog("divider.session.mouseDown loc=\(Int(location.x)),\(Int(location.y))")
+        }
+#endif
+        if inDivider { onDividerDragSession?(true) }
+        // For a divider hit, super runs AppKit's tracking loop and returns
+        // only after the mouse is released — the session end below is the
+        // guaranteed drag-end signal.
+        defer { if inDivider { onDividerDragSession?(false) } }
+        super.mouseDown(with: event)
+    }
 }
 
 #if DEBUG
@@ -213,6 +247,21 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
         context.coordinator.secondHostingController = secondController
 
         context.coordinator.splitView = splitView
+        let internalController = controller
+        splitView.onDividerDragSession = { [weak coordinator = context.coordinator, weak internalController] active in
+            // Order matters at session END: the counter must drop to zero
+            // BEFORE the coordinator's final geometry notification, or the
+            // delegate still reads the drag as live and skips the drag-end
+            // sync. At BEGIN the coordinator arms first so isDragging is set
+            // before any delegate reacts to the session.
+            if active {
+                coordinator?.dividerDragSessionChanged(true)
+                internalController?.noteDividerDragSession(true)
+            } else {
+                internalController?.noteDividerDragSession(false)
+                coordinator?.dividerDragSessionChanged(false)
+            }
+        }
 
         // Capture animation origin before it gets cleared
         let animationOrigin = splitState.animationOrigin
@@ -604,12 +653,12 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
         var onGeometryChange: ((_ isDragging: Bool) -> Void)?
         /// Track last applied position to detect external changes
         var lastAppliedPosition: CGFloat = 0.5
-        /// The imposed-extent memo: the last target we asked AppKit for and
-        /// the position it actually produced. Re-apply only when the target
-        /// changes or the divider moved away from that outcome — never in a
-        /// loop against a target AppKit's constraints refuse to reach.
-        var lastImposedTarget: CGFloat?
+        /// What the last imposed apply actually produced, and how big the
+        /// split view was at the time. syncPosition compares against both to
+        /// decide whether a moved divider should be put back or left for the
+        /// host to re-impose at the new size.
         var lastImposedOutcome: CGFloat?
+        var lastImposedAvail: CGFloat?
         var lastImposedEpoch: Int?
         var imposedRetryBudget = 0
         var imposedApplyPending = false
@@ -661,8 +710,8 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
                 splitStateId = newState.id
                 splitState = newState
                 lastAppliedPosition = newState.dividerPosition
-                lastImposedTarget = nil
                 lastImposedOutcome = nil
+                lastImposedAvail = nil
                 lastImposedEpoch = nil
                 imposedRetryBudget = 0
                 didApplyInitialDividerPosition = false
@@ -678,6 +727,25 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
             splitState = newState
         }
 
+        /// Deterministic drag session, bracketed by `ThemedSplitView.mouseDown`
+        /// around AppKit's divider tracking loop. Begin arms the drag before
+        /// any resize callback needs to infer it; end always fires at release
+        /// and delivers the drag-end geometry notification — whether or not a
+        /// final resize callback coincided with the mouseUp. The model itself
+        /// is maintained by the resize callbacks during the drag; end only has
+        /// to announce that the user's hand is off the divider.
+        func dividerDragSessionChanged(_ active: Bool) {
+#if DEBUG
+            dlog("divider.session split=\(String(splitState.id.uuidString.prefix(5))) \(active ? "begin" : "end")")
+#endif
+            if active {
+                isDragging = true
+                return
+            }
+            isDragging = false
+            onGeometryChange?(false)
+        }
+
         private func splitTotalSize(in splitView: NSSplitView) -> CGFloat {
             splitState.orientation == .horizontal
                 ? splitView.bounds.width
@@ -791,6 +859,17 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
             lastImposedOutcome = splitState.orientation == .horizontal
                 ? splitView.arrangedSubviews[0].frame.width
                 : splitView.arrangedSubviews[0].frame.height
+            // When this retry finally lands, it resizes everything nested
+            // inside — AFTER those nested splits already applied their own
+            // extents and recorded the result. AppKit resizes them
+            // proportionally, so their dividers end up off the extents they
+            // were given, and nothing else would ever correct that. Ask each
+            // nested imposed split to apply its extent again now that its
+            // container has settled. Runs at most once per retry, and
+            // retries are budgeted.
+            if abs((lastImposedOutcome ?? target) - target) <= 0.01 {
+                renudgeImposedDescendants(of: splitState)
+            }
 #if DEBUG
             dlog(
                 "bonsplit.impose.retry split=\(String(splitState.id.uuidString.prefix(5)))"
@@ -803,6 +882,17 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
             }
         }
 
+        private func renudgeImposedDescendants(of state: SplitState) {
+            for child in [state.first, state.second] {
+                guard case .split(let childState) = child else { continue }
+                if childState.imposedFirstExtent != nil {
+                    childState.imposedEpoch &+= 1
+                    childState.applyImposedNow?()
+                }
+                renudgeImposedDescendants(of: childState)
+            }
+        }
+
         func setPositionSafely(_ position: CGFloat, in splitView: NSSplitView, layout: Bool = true) {
 #if DEBUG
             // Wobble hunt: name every divider write while an imposition is
@@ -874,39 +964,47 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
                 // `current != target` then re-layouts on every pass — a
                 // main-thread spin. Re-apply only when the target changed or
                 // something ELSE moved the divider off our last outcome.
-                let moved = abs(current - (lastImposedOutcome ?? .infinity)) > 0.01
-                let retargeted = abs(target - (lastImposedTarget ?? .infinity)) > 0.01
                 // A fresh imposition call re-arms one apply attempt even for
                 // an identical target: AppKit may have refused this exact
                 // target earlier (transient pane minimums mid-churn), and
                 // with neither target nor divider moving since, nothing else
                 // would ever retry — panes would sit wedged at the refused
                 // layout. Bounded by explicit calls, so it cannot spin.
                 let renudged = lastImposedEpoch != splitState.imposedEpoch
-                // Never apply when the divider already sits at the target:
-                // a same-position setPosition still runs a layout pass,
-                // which re-applies surface sizes, which re-imposes — a
-                // sustained once-per-turn churn loop at full CPU. Renudge
-                // and retarget only bypass the refusal memo; distance from
-                // the target is what justifies touching AppKit.
+                // A new imposition always applies. Beyond that, when the
+                // divider is not where we left it, what to do depends on
+                // whether the split view itself was resized since we last
+                // applied. If it was, our stored extent is out of date and
+                // the host will send a new one computed for the new size —
+                // re-applying the old one here just moves the divider back
+                // and forth against AppKit on every update until it does.
+                // If the split view is the SAME size, no new extent is
+                // coming (the host only recomputes when something it can see
+                // changed), so a nudged divider would stay wrong forever —
+                // put it back; one apply settles it, since applying cannot
+                // resize the split view. And never apply when the divider is
+                // already at the target: a same-position setPosition still
+                // runs a layout pass, which re-applies surface sizes, which
+                // re-imposes — a once-per-turn churn loop at full CPU.
+                let moved = abs(current - (lastImposedOutcome ?? .infinity)) > 0.01
+                let availUnchanged = abs(available - (lastImposedAvail ?? -1)) <= 0.01
                 if abs(current - target) <= 0.01 {
-                    lastImposedTarget = target
                     lastImposedEpoch = splitState.imposedEpoch
                     lastImposedOutcome = current
+                    lastImposedAvail = available
                     imposedRetryBudget = 0
-                } else if renudged || retargeted || moved {
+                } else if renudged || (moved && availUnchanged) {
                     setPositionSafely(target, in: splitView, layout: true)
-                    lastImposedTarget = target
                     lastImposedEpoch = splitState.imposedEpoch
+                    lastImposedAvail = available
                     lastImposedOutcome = splitState.orientation == .horizontal
                         ? splitView.arrangedSubviews[0].frame.width
                         : splitView.arrangedSubviews[0].frame.height
 #if DEBUG
                     dlog(
                         "bonsplit.impose split=\(String(splitState.id.uuidString.prefix(5)))"
                             + " target=\(Int(target)) outcome=\(Int(lastImposedOutcome ?? -1))"
-                            + " avail=\(Int(available)) renudged=\(renudged ? 1 : 0)"
-                            + " retargeted=\(retargeted ? 1 : 0) moved=\(moved ? 1 : 0)"
+                            + " avail=\(Int(available))"
                     )
 #endif
                     // A refused apply (AppKit clamped against constraints
@@ -1161,12 +1259,22 @@ struct SplitContainerView<Content: View, EmptyContent: View>: NSViewRepresentabl
                         "divider.resizeIgnored split=\(splitState.id.uuidString.prefix(5)) eventType=\(eventType) leftDown=\(leftDown ? 1 : 0) isDragging=\(isDragging ? 1 : 0) normalized=\(String(format: "%.3f", normalizedPosition)) model=\(String(format: "%.3f", self.splitState.dividerPosition))"
                     )
 #endif
-                    let statePosition = self.splitState.dividerPosition
-                    // Re-assert synchronously. setPositionSafely sets isSyncingProgrammatically=true,
-                    // so the recursive splitViewDidResizeSubviews call is caught by the guard above.
-                    // Deferring to the next runloop turn would allow the transient frame to propagate
-                    // through SwiftUI layout → ghostty terminal resize → reflow, causing content shifts.
-                    self.syncPosition(statePosition, in: splitView)
+                    // A split the user positions by fraction puts its divider
+                    // back right here, synchronously (setPositionSafely sets
+                    // isSyncingProgrammatically, so the recursive didResize is
+                    // caught by the guard above; waiting a turn would let the
+                    // in-between frame reach ghostty and reflow content). A
+                    // split with an imposed extent must NOT do that: when its
+                    // container resizes, the stored extent is out of date, and
+                    // putting the divider back from inside the very layout
+                    // pass that moved it starts another layout pass — that
+                    // recursion is what pinned the main thread. The host sends
+                    // a new extent for the new size; until it lands, a
+                    // not-yet-right frame is fine.
+                    if self.splitState.imposedFirstExtent == nil {
+                        let statePosition = self.splitState.dividerPosition
+                        self.syncPosition(statePosition, in: splitView)
+                    }
                     self.onGeometryChange?(false)
                     return
                 }

Sources/Bonsplit/Public/BonsplitController.swift

@@ -118,6 +118,30 @@ public final class BonsplitController {
     public init(configuration: BonsplitConfiguration = .default) {
         self.configuration = configuration
         self.internalController = SplitViewController()
+        internalController.onDividerDragSessionChange = { [weak self] active in
+            guard let self else { return }
+            if active {
+                self.delegate?.splitTabBarDividerDragDidBegin(self)
+            } else {
+                self.delegate?.splitTabBarDividerDragDidEnd(self)
+            }
+        }
+    }
+
+    /// True while the user is dragging a divider anywhere in this tree.
+    /// Sessions bracket the divider's mouse-tracking lifecycle, so this is
+    /// authoritative — not inferred from resize callbacks. While true,
+    /// external sizing must not impose extents: the drag owns geometry.
+    public var isDividerDragActive: Bool {
+        internalController.activeDividerDragSessions > 0
+    }
+
+    /// Brackets a divider drag session from outside the built-in divider
+    /// tracking — for hosts with custom drag surfaces, and for tests that
+    /// need the session state without synthesizing mouse events. Balanced
+    /// begin/end pairs are the caller's responsibility.
+    public func noteDividerDragSession(_ active: Bool) {
+        internalController.noteDividerDragSession(active)
     }
 
     // MARK: - Tab Operations
@@ -992,6 +1016,15 @@ public final class BonsplitController {
     /// the split to fraction semantics. Use this when pane contents are
     /// grid-quantized (terminal cells) and a normalized fraction cannot
     /// express the required pixel size losslessly.
+    ///
+    /// The extent is applied when set (and re-applied if the divider drifts
+    /// while the split view's own size is unchanged), but it is NOT
+    /// re-asserted after the split view resizes: a stored extent computed
+    /// for the old size is stale at the new one, and re-applying it from
+    /// inside the resize's layout pass fights AppKit recursively. A host
+    /// that imposes extents is expected to impose fresh values when the
+    /// container it derived them from changes; until it does, the divider
+    /// rides AppKit's proportional resize.
     public func setImposedFirstExtent(
         _ extent: CGFloat?, forSplit splitId: UUID, fromExternal: Bool = false
     ) -> Bool {

Sources/Bonsplit/Public/BonsplitDelegate.swift

@@ -76,6 +76,17 @@ public protocol BonsplitDelegate: AnyObject {
 
     /// Called to check if notifications should be sent during divider drag (opt-in for real-time sync)
     func splitTabBar(_ controller: BonsplitController, shouldNotifyDuringDrag: Bool) -> Bool
+
+    /// A divider drag session started: the mouse went down on a divider and
+    /// AppKit's tracking loop is running. Until the matching end, the drag
+    /// owns divider geometry — hosts that impose extents should defer.
+    func splitTabBarDividerDragDidBegin(_ controller: BonsplitController)
+
+    /// The divider drag session ended (mouse released). Delivered from the
+    /// tracking lifecycle itself, so it fires even when no resize callback
+    /// coincides with the release; the final geometry has already been
+    /// reported through `didChangeGeometry`.
+    func splitTabBarDividerDragDidEnd(_ controller: BonsplitController)
 }
 
 // MARK: - Default Implementations (all methods optional)
@@ -99,4 +110,6 @@ public extension BonsplitDelegate {
     func splitTabBar(_ controller: BonsplitController, didRequestTabMoveToDestination destinationId: String, for tab: Tab, inPane pane: PaneID) {}
     func splitTabBar(_ controller: BonsplitController, didChangeGeometry snapshot: LayoutSnapshot) {}
     func splitTabBar(_ controller: BonsplitController, shouldNotifyDuringDrag: Bool) -> Bool { false }
+    func splitTabBarDividerDragDidBegin(_ controller: BonsplitController) {}
+    func splitTabBarDividerDragDidEnd(_ controller: BonsplitController) {}
 }