Implement SwiftUI's layout system (worse behaviour, but better performance)

Some of the bad behaviour from SwiftUI's layout system can probably be
mitigated, but other undesirable behaviour is probably the best we can
do, cause some things are impossible to compute nicely without laying
out child views more than once (a no go for performant layout)

Author: stackotter

Benchmarks/LayoutPerformanceBenchmark/LayoutPerformanceBenchmark.swift

@@ -34,7 +34,7 @@ struct Benchmarks {
         }
 
         @MainActor
-        func updateNode<V: View>(_ node: ViewGraphNode<V, DummyBackend>, _ size: SIMD2<Int>) {
+        func updateNode<V: View>(_ node: ViewGraphNode<V, DummyBackend>, _ size: ProposedViewSize) {
             _ = node.computeLayout(proposedSize: size, environment: environment)
             _ = node.commit()
         }
@@ -44,7 +44,7 @@ struct Benchmarks {
         #endif
 
         @MainActor
-        func benchmarkLayout<V: TestCaseView>(of viewType: V.Type, _ size: SIMD2<Int>, _ label: String) {
+        func benchmarkLayout<V: TestCaseView>(of viewType: V.Type, _ size: ProposedViewSize, _ label: String) {
             #if BENCHMARK_VIZ
                 benchmarkVisualizations.append((
                     label,
@@ -62,8 +62,8 @@ struct Benchmarks {
         }
 
         // Register benchmarks
-        benchmarkLayout(of: GridView.self, SIMD2(800, 800), "grid")
-        benchmarkLayout(of: ScrollableMessageListView.self, SIMD2(800, 800), "message list")
+        benchmarkLayout(of: GridView.self, ProposedViewSize(800, 800), "grid")
+        benchmarkLayout(of: ScrollableMessageListView.self, ProposedViewSize(800, 800), "message list")
 
         #if BENCHMARK_VIZ
             let names = benchmarkVisualizations.map(\.name).joined(separator: " | ")

Sources/AppKitBackend/AppKitBackend.swift

@@ -530,7 +530,7 @@ public final class AppKitBackend: AppBackend {
     ) -> SIMD2<Int> {
         if let proposedFrame, proposedFrame.x == 0 {
             // We want the text to have the same height as it would have if it were
-            // one pixel wide so that the layout doesn't suddely jump when the text
+            // one pixel wide so that the layout doesn't suddenly jump when the text
             // reaches zero width.
             let size = size(
                 of: text,