fix #387: window resize never recreated the Vulkan swapchain (stretch)

User report: resolution picks (and window drags) stretched the image
instead of changing the pixel count. Root cause: Campaign V slice V11
deleted the GL viewport target and left a null target, assuming the
driver's OUT_OF_DATE/SUBOPTIMAL acquire/present results would drive
swapchain recreation on resize. That is driver-dependent and
spec-insufficient — this machine's Windows AMD driver keeps presenting
the stale-extent swapchain scaled to the new window indefinitely, so
OnFramebufferResize only ever updated the camera aspect while every
pass (UI included) kept rendering at the old extent.

Fix: SwapchainRecreateViewportTarget implements the existing
IFramebufferViewportTarget seam for Vulkan and arms
VulkanGraphicsContext.RequestRecreate() on every resize event; the next
PrepareFrame rebuilds the swapchain at the live FramebufferSize (bursts
collapse to one recreation, stale events cannot install a stale extent,
minimised sizes stay gated by FramebufferResizeController).

Tests: SwapchainRecreateViewportTargetTests (target contract, size-
agnostic arming, null hook, controller-to-target end-to-end with the
minimised gate). Full Debug App suite 4,941/3 skips.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ISSUES.md

@@ -24,6 +24,38 @@ What does NOT go here:
 - Every session: scan OPEN issues at start; promote/close anything we touched during the session before ending.
 - Promoting to a Phase: mark as `DONE (promoted to Phase X)` + commit SHA where the Phase entry landed.
 
+## #387 — Window resize never recreates the Vulkan swapchain: resolution picks stretch the image instead of changing the pixel count
+
+**Status:** DONE 2026-08-13 (this commit) — pending the user's re-check.
+User report (verbatim shape): "It looks like it is doing now is just
+stretching the window, not changing the pixel count when I change the
+resolution." Confirmed real and root-caused the same session.
+
+**ROOT CAUSE — the resize event never reached the swapchain.** Campaign V
+slice V11 deleted the GL `SilkFramebufferViewportTarget` and left
+`NullFramebufferViewportTarget` on the assumption that the driver's
+OUT_OF_DATE/SUBOPTIMAL acquire/present results would drive
+`VulkanGraphicsContext`'s frame-boundary swapchain recreation whenever the
+window resized. That assumption is driver-dependent and spec-insufficient:
+a conformant driver may keep presenting the stale-extent swapchain scaled
+to the new window indefinitely — which is exactly what this machine's
+Windows AMD driver does. Result: `OnFramebufferResize` updated only the
+camera aspect; the swapchain (and every render pass sized from its extent,
+UI included) stayed at the old pixel count and the presentation engine
+stretched it — for Options resolution picks AND manual window-edge drags
+alike. **Fix:** `SwapchainRecreateViewportTarget` (the Vulkan
+implementation of the existing `IFramebufferViewportTarget` seam) arms
+`VulkanGraphicsContext.RequestRecreate()` on every resize event; the next
+`PrepareFrame` rebuilds the swapchain at the live `FramebufferSize`, so
+event bursts collapse into one recreation and stale events cannot install
+a stale extent. Regressed by
+`tests/AcDream.App.Tests/Composition/SwapchainRecreateViewportTargetTests.cs`
+(target contract + the controller→target end-to-end seam with the
+minimised-gate case). **Re-check:** pick a smaller Resolution — the window
+should shrink AND the image should re-render crisp at the new pixel count
+(UI elements occupy proportionally more of the window, retail-style), not
+scale down blurrily; same for a window-edge drag.
+
 ## #386 — Vendor category dropdown: authored ListBox is edge-docked — retail would size the popup to content, our shipped 6-row window may diverge
 
 **Status:** OPEN — filed 2026-08-13 while fixing #385. The #385 probe

src/AcDream.App/Composition/VulkanHostInputCameraCompositionFactory.cs

@@ -26,11 +26,17 @@ internal sealed class VulkanHostInputCameraCompositionFactory
 {
     public IFramebufferViewportTarget CreateViewportTarget(
         GameWindowGraphics graphics) =>
-        // The viewport is a pipeline dynamic state on Vulkan, set per pass by
-        // the encoder from the pass extent, so there is no persistent viewport
-        // to bind here. The framebuffer-resize controller still drives the
-        // camera aspect through its own target.
-        NullFramebufferViewportTarget.Instance;
+        // The viewport itself is a pipeline dynamic state on Vulkan, set per
+        // pass by the encoder from the pass extent — but the swapchain still
+        // has to be told the framebuffer changed. Relying on the driver's
+        // OUT_OF_DATE/SUBOPTIMAL results alone is spec-insufficient: a
+        // conformant driver (observed: Windows AMD, 2026-08-13 user gate —
+        // "just stretching the window, not changing the pixel count") may keep
+        // presenting the stale-extent swapchain scaled to the new window
+        // forever. The V11 null target assumed the driver signal; this target
+        // arms the context's existing frame-boundary recreation instead, so
+        // the next PrepareFrame recreates at the real FramebufferSize.
+        new SwapchainRecreateViewportTarget(RequireContext(graphics).RequestRecreate);
 
     public GpuFrameFlightController? CreateGpuFrameFlights(
         GameWindowGraphics graphics) => null;
@@ -110,22 +116,24 @@ internal sealed class VulkanHostInputCameraCompositionFactory
             "Vulkan context.");
 
     /// <summary>
-    /// Vulkan sets the viewport per pass from the pass extent, so there is no
-    /// persistent viewport binding for the resize controller to update. Size is
-    /// still recorded, because the swapchain recreation the host performs is
-    /// what actually resizes the surface.
+    /// The Vulkan implementation of the resize seam (#387): a window resize
+    /// arms <see cref="VulkanGraphicsContext.RequestRecreate"/> so the next
+    /// <c>PrepareFrame</c> rebuilds the swapchain at the current
+    /// FramebufferSize. The width/height arguments are deliberately unused —
+    /// the context re-reads the live framebuffer size at the frame boundary,
+    /// so a burst of resize events collapses into one recreation and a stale
+    /// event can never install a stale extent. Zero-area (minimised) sizes
+    /// never reach this target (<see cref="FramebufferResizeController.Resize(int,int)"/>
+    /// gates them), and the armed flag survives a minimised skip until a
+    /// non-zero frame runs, which is the context's own existing behavior.
     /// </summary>
-    private sealed class NullFramebufferViewportTarget : IFramebufferViewportTarget
+    internal sealed class SwapchainRecreateViewportTarget(Action requestRecreate)
+        : IFramebufferViewportTarget
     {
-        public static NullFramebufferViewportTarget Instance { get; } = new();
+        private readonly Action _requestRecreate = requestRecreate
+            ?? throw new ArgumentNullException(nameof(requestRecreate));
 
-        private NullFramebufferViewportTarget()
-        {
-        }
-
-        public void ResizeViewport(int width, int height)
-        {
-        }
+        public void ResizeViewport(int width, int height) => _requestRecreate();
     }
 
     /// <summary>

src/AcDream.App/Rendering/FramebufferResizeController.cs

@@ -9,10 +9,15 @@ internal interface IFramebufferViewportTarget
 }
 
 // Campaign V slice V11 deleted SilkFramebufferViewportTarget, the GL
-// implementation of IFramebufferViewportTarget: VulkanHostInputCameraComposition
-// Factory's NullFramebufferViewportTarget is the sole surviving implementation
-// (Vulkan's swapchain recreation owns the actual viewport-equivalent resize,
-// which this seam never drove).
+// implementation of IFramebufferViewportTarget, and left a null target on the
+// assumption that the driver's OUT_OF_DATE/SUBOPTIMAL results would drive
+// swapchain recreation on resize. That assumption is driver-dependent — a
+// conformant driver may present a stale-extent swapchain scaled to the new
+// window indefinitely (observed on Windows AMD; #387, the 2026-08-13
+// "resolution pick just stretches" user gate report). The sole implementation
+// is now VulkanHostInputCameraCompositionFactory.SwapchainRecreateViewportTarget,
+// which arms the context's frame-boundary swapchain recreation on every
+// resize event.
 
 internal interface IFramebufferCameraTarget
 {

tests/AcDream.App.Tests/Composition/SwapchainRecreateViewportTargetTests.cs

@@ -0,0 +1,80 @@
+using AcDream.App.Composition;
+using AcDream.App.Input;
+using AcDream.App.Rendering;
+
+namespace AcDream.App.Tests.Composition;
+
+/// <summary>
+/// #387: the V11 null viewport target assumed the driver's OUT_OF_DATE/
+/// SUBOPTIMAL results would drive swapchain recreation on window resize; on
+/// Windows AMD the driver instead presents the stale-extent swapchain scaled
+/// to the new window (the "resolution pick just stretches" 2026-08-13 user
+/// gate report). These pin the replacement target's contract: every resize
+/// event reaching the seam arms the context's frame-boundary recreation.
+/// </summary>
+public sealed class SwapchainRecreateViewportTargetTests
+{
+    [Fact]
+    public void ResizeViewport_ArmsRecreation_OncePerEvent()
+    {
+        int armed = 0;
+        IFramebufferViewportTarget target =
+            new VulkanHostInputCameraCompositionFactory.SwapchainRecreateViewportTarget(
+                () => armed++);
+
+        target.ResizeViewport(1024, 768);
+        Assert.Equal(1, armed);
+
+        // A second resize arms again — the context collapses bursts itself at
+        // the frame boundary (it re-reads the live FramebufferSize), so the
+        // target must never de-duplicate or latch.
+        target.ResizeViewport(800, 600);
+        Assert.Equal(2, armed);
+    }
+
+    [Fact]
+    public void ResizeViewport_IgnoresTheEventSize_ContextReadsLiveFramebuffer()
+    {
+        // The width/height are deliberately unused (doc on the target): the
+        // context reads the CURRENT framebuffer size at the frame boundary,
+        // so a stale event cannot install a stale extent. Pin that the arm
+        // fires regardless of the reported size values.
+        int armed = 0;
+        IFramebufferViewportTarget target =
+            new VulkanHostInputCameraCompositionFactory.SwapchainRecreateViewportTarget(
+                () => armed++);
+
+        target.ResizeViewport(int.MaxValue, 1);
+        Assert.Equal(1, armed);
+    }
+
+    [Fact]
+    public void Constructor_RejectsNullRecreateHook()
+    {
+        Assert.Throws<ArgumentNullException>(() =>
+            new VulkanHostInputCameraCompositionFactory.SwapchainRecreateViewportTarget(null!));
+    }
+
+    [Fact]
+    public void ResizeController_DeliversResizeEventsToTheBoundTarget()
+    {
+        // The end-to-end seam FramebufferResizeController.Resize →
+        // ResizeViewport → arm: the same event flow GameWindow's
+        // OnFramebufferResize drives in production. Zero-area (minimised)
+        // events are gated by the controller and must NOT arm.
+        int armed = 0;
+        var controller = new FramebufferResizeController(new ViewportAspectState());
+        controller.BindViewport(
+            new VulkanHostInputCameraCompositionFactory.SwapchainRecreateViewportTarget(
+                () => armed++));
+
+        controller.Resize(1280, 720);
+        Assert.Equal(1, armed);
+
+        controller.Resize(0, 0);       // minimised — gated, no arm
+        Assert.Equal(1, armed);
+
+        controller.Resize(1024, 768);
+        Assert.Equal(2, armed);
+    }
+}