fix(gui): resolve D3D11 image preview jitter during horizontal resize

- Remove Flush() before ResizeBuffers — eliminates CPU stall that
  widens the window where DWM composites stale front buffer
- Use event-provided pixel dimensions directly instead of querying
  SDL_GetWindowSizeInPixels() which may return stale values
- Skip vsync (Present(0,0)) on resize frames so new content reaches
  DWM before next composition cycle
- Add same-size early return to avoid redundant ResizeBuffers calls
- Only handle SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED (physical pixels),
  drop SDL_EVENT_WINDOW_RESIZED (logical size, wrong on HiDPI)

Root cause: DXGI_SCALING_NONE + ResizeBuffers destroys all back buffers,
leaving a timing gap where DWM composites the stale front buffer at
the old size, top-left aligned in the new window — visible as a
per-frame position oscillation. OpenGL unaffected (glViewport never
invalidates framebuffer content).

Author: allenk

src/gui/backend/d3d11_backend.cpp

@@ -403,21 +403,26 @@ void D3D11Backend::end_frame() {
 }
 
 void D3D11Backend::present() {
-    // Present with vsync
-    m_swap_chain->Present(1, 0);
+    // During resize: present immediately (sync=0) to push frame to DWM before
+    // it composites stale content. Normal frames use vsync (sync=1).
+    UINT sync_interval = m_in_resize ? 0 : 1;
+    m_swap_chain->Present(sync_interval, 0);
+    m_in_resize = false;
 }
 
 void D3D11Backend::on_resize(int width, int height) {
     if (!m_initialized || width <= 0 || height <= 0) return;
+    if (m_window_width == width && m_window_height == height) return;
 
-    // Get actual drawable size
-    SDL_GetWindowSizeInPixels(m_window, &m_window_width, &m_window_height);
+    m_window_width = width;
+    m_window_height = height;
 
-    // Release render target before resize
+    // Release render target before resize (required by DXGI)
     cleanup_render_target();
-    m_context->Flush();
 
-    // Resize swap chain buffers — flags = 0 (cooperative, no mode switch)
+    // Resize swap chain buffers — no Flush() needed; ResizeBuffers handles
+    // outstanding GPU work internally. Removing Flush() avoids a CPU stall
+    // that widens the window where DWM composites stale content.
     HRESULT hr = m_swap_chain->ResizeBuffers(
         0,                      // Keep current buffer count
         m_window_width,
@@ -434,6 +439,10 @@ void D3D11Backend::on_resize(int width, int height) {
 
     // Recreate render target
     create_render_target();
+
+    // Signal present() to use immediate mode (skip vsync) so the resized
+    // frame reaches DWM before it composites the stale front buffer.
+    m_in_resize = true;
 }
 
 // =============================================================================

src/gui/backend/d3d11_backend.hpp

@@ -124,6 +124,7 @@ class D3D11Backend final : public IRenderBackend {
     int m_window_height{0};
     bool m_initialized{false};
     bool m_using_warp{false};
+    bool m_in_resize{false};  // Set during resize → present without vsync
 
     // Feature level info
     D3D_FEATURE_LEVEL m_feature_level{D3D_FEATURE_LEVEL_11_0};

src/gui/gui_app.cpp

@@ -384,13 +384,11 @@ int run(int argc, char** argv) {
     auto event_watch = [](void* userdata, SDL_Event* event) -> bool {
         auto* ctx = static_cast<RenderContext*>(userdata);
 
-        if (event->type == SDL_EVENT_WINDOW_RESIZED ||
-            event->type == SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED ||
+        if (event->type == SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED ||
             event->type == SDL_EVENT_WINDOW_EXPOSED) {
 
-            // Update backend size
-            if (event->type == SDL_EVENT_WINDOW_RESIZED ||
-                event->type == SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED) {
+            // Update backend with pixel-accurate size (not logical size from RESIZED)
+            if (event->type == SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED) {
                 ctx->backend->on_resize(event->window.data1, event->window.data2);
             }
 
@@ -444,9 +442,8 @@ int run(int argc, char** argv) {
                     }
                     break;
 
-                case SDL_EVENT_WINDOW_RESIZED:
                 case SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED:
-                    // Resize is handled in event watch callback
+                    // Sync backend to current pixel size
                     backend->on_resize(event.window.data1, event.window.data2);
                     break;