From 409246f7adee3b42ca9ec2c1dbf5e4f6af2ad78c Mon Sep 17 00:00:00 2001
From: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Date: Tue, 28 Apr 2026 23:36:36 +0000
Subject: [PATCH] =?UTF-8?q?perf(layout):=20override=20explicitAlignment=20?=
 =?UTF-8?q?to=20stop=20O(n=C3=97depth)=20cascade=20in=20custom=20Layout=20?=
 =?UTF-8?q?wrappers=20(LUM-1167)?=
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BottomAlignedMinHeightLayout and WidthCapLayout were introduced to replace
_FlexFrameLayout, but neither overrode Layout.explicitAlignment. The default
protocol extension merges all subviews' alignment guides — the exact same
O(n×depth) recursive cascade that _FlexFrameLayout performs. When
BottomAlignedMinHeightLayout wraps the entire LazyVStack scroll content,
this cascade walks every visible cell on each layout pass, producing
2000ms+ main-thread hangs (Sentry MACOS-M9, MACOS-JG).

Override both explicitAlignment(of: HorizontalAlignment) and
explicitAlignment(of: VerticalAlignment) to return nil on both layouts.
Returning nil means 'no explicit guide — use default positioning', which
is correct because both layouts position children via placeSubviews, not
alignment guides.

Closes LUM-1167

Co-Authored-By: ashlee@vellum.ai <ashlee@vellum.ai>
---
 .../BottomAlignedMinHeightLayout.swift        | 22 +++++++++++++++++++
 .../Modifiers/WidthCapLayout.swift            | 19 ++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/clients/shared/DesignSystem/Modifiers/BottomAlignedMinHeightLayout.swift b/clients/shared/DesignSystem/Modifiers/BottomAlignedMinHeightLayout.swift
index 6c7b594e32a..38dadc472d2 100644
--- a/clients/shared/DesignSystem/Modifiers/BottomAlignedMinHeightLayout.swift
+++ b/clients/shared/DesignSystem/Modifiers/BottomAlignedMinHeightLayout.swift
@@ -46,6 +46,28 @@ public struct BottomAlignedMinHeightLayout: Layout {
             proposal: ProposedViewSize(width: childSize.width, height: childSize.height)
         )
     }
+
+    // MARK: - Alignment (opt out of default cascade)
+
+    /// Returns `nil` to opt out of the default guide-merging cascade.
+    ///
+    /// The default `Layout` protocol implementation iterates every subview
+    /// and recursively queries their alignment guides — O(n × depth). When
+    /// this layout wraps the entire LazyVStack scroll content, the cascade
+    /// walks every visible cell, producing multi-second hangs.
+    ///
+    /// Returning `nil` tells ancestors "no explicit guide value; use default
+    /// positioning", which is correct because this layout positions its
+    /// child via `placeSubviews`, not alignment guides.
+    ///
+    /// Reference: [Layout.explicitAlignment](https://developer.apple.com/documentation/swiftui/layout/explicitalignment(of:in:proposal:subviews:cache:)-8ofeu)
+    public func explicitAlignment(of guide: HorizontalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGFloat? {
+        nil
+    }
+
+    public func explicitAlignment(of guide: VerticalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGFloat? {
+        nil
+    }
 }
 
 extension View {
diff --git a/clients/shared/DesignSystem/Modifiers/WidthCapLayout.swift b/clients/shared/DesignSystem/Modifiers/WidthCapLayout.swift
index 016cb1f3b14..8c90f64a37e 100644
--- a/clients/shared/DesignSystem/Modifiers/WidthCapLayout.swift
+++ b/clients/shared/DesignSystem/Modifiers/WidthCapLayout.swift
@@ -29,6 +29,25 @@ public struct WidthCapLayout: Layout {
             proposal: ProposedViewSize(width: bounds.width, height: bounds.height)
         )
     }
+
+    // MARK: - Alignment (opt out of default cascade)
+
+    /// Returns `nil` to opt out of the default guide-merging cascade.
+    ///
+    /// The default `Layout` protocol implementation iterates every subview
+    /// and recursively queries their alignment guides — O(n × depth).
+    /// Returning `nil` tells ancestors "no explicit guide value; use default
+    /// positioning", which is correct because this layout positions its
+    /// child via `placeSubviews`, not alignment guides.
+    ///
+    /// Reference: [Layout.explicitAlignment](https://developer.apple.com/documentation/swiftui/layout/explicitalignment(of:in:proposal:subviews:cache:)-8ofeu)
+    public func explicitAlignment(of guide: HorizontalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGFloat? {
+        nil
+    }
+
+    public func explicitAlignment(of guide: VerticalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGFloat? {
+        nil
+    }
 }
 
 extension View {