diff --git a/src/main.cpp b/src/main.cpp
index ff66bc2..9e3418a 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -23,6 +23,7 @@
 #include "imgui_internal.h"
 #include "imgui_impl_sdl3.h"
 #include "ui/effects/imgui_acrylic.h"
+#include "ui/effects/capture_extent.h"
 #include "ui/effects/theme_effects.h"
 #include "util/texture_loader.h"
 #include "util/platform.h"
@@ -1557,7 +1558,9 @@ int main(int argc, char* argv[])
 
         ImGuiIO& io = ImGui::GetIO();
         dragonx::ui::effects::ImGuiAcrylic::BeginFrame(
-            (int)io.DisplaySize.x, (int)io.DisplaySize.y);
+            (int)io.DisplaySize.x, (int)io.DisplaySize.y,
+            dragonx::ui::effects::framebufferExtent(io.DisplaySize.x, io.DisplayFramebufferScale.x),
+            dragonx::ui::effects::framebufferExtent(io.DisplaySize.y, io.DisplayFramebufferScale.y));
 
         ctx->app->update();
         ctx->app->render();
@@ -2049,7 +2052,9 @@ int main(int argc, char* argv[])
         ImGuiIO& io = ImGui::GetIO();
         if (!dragonx::ui::effects::isLowSpecMode()) {
             dragonx::ui::effects::ImGuiAcrylic::BeginFrame(
-                (int)io.DisplaySize.x, (int)io.DisplaySize.y);
+                (int)io.DisplaySize.x, (int)io.DisplaySize.y,
+                dragonx::ui::effects::framebufferExtent(io.DisplaySize.x, io.DisplayFramebufferScale.x),
+                dragonx::ui::effects::framebufferExtent(io.DisplaySize.y, io.DisplayFramebufferScale.y));
         }
 
         // Update theme visual effects timing (held still under Reduce Motion)
@@ -2213,7 +2218,9 @@ int main(int argc, char* argv[])
 
         // Debug screenshot sweep — grab the finished frame before swap.
         if (app.wantsScreenshotThisFrame()) {
-            captureFrameToPng(app.screenshotSweepPath(), (int)io.DisplaySize.x, (int)io.DisplaySize.y);
+            captureFrameToPng(app.screenshotSweepPath(),
+                dragonx::ui::effects::framebufferExtent(io.DisplaySize.x, io.DisplayFramebufferScale.x),
+                dragonx::ui::effects::framebufferExtent(io.DisplaySize.y, io.DisplayFramebufferScale.y));
             app.onScreenshotCaptured();
         }
 
diff --git a/src/ui/effects/acrylic.cpp b/src/ui/effects/acrylic.cpp
index 93434b8..dcb5b7f 100644
--- a/src/ui/effects/acrylic.cpp
+++ b/src/ui/effects/acrylic.cpp
@@ -198,7 +198,7 @@ void AcrylicMaterial::captureBackground()
     --dirtyFrames_;
     
     // Capture current screen content
-    captureBuffer_.captureScreen(viewportWidth_, viewportHeight_);
+    captureBuffer_.captureScreen(captureSrcWidth(), captureSrcHeight());
     hasValidCapture_ = true;
     blurCacheValid_ = false;  // New capture invalidates blur cache
 }
@@ -226,7 +226,7 @@ void AcrylicMaterial::captureBackgroundDirect()
     // draw framebuffer, and ImGui has scissor enabled during rendering.
     // The subsequent ImDrawCallback_ResetRenderState re-enables it.
     glDisable(GL_SCISSOR_TEST);
-    captureBuffer_.captureScreen(viewportWidth_, viewportHeight_);
+    captureBuffer_.captureScreen(captureSrcWidth(), captureSrcHeight());
 
     hasValidCapture_ = true;
     blurCacheValid_ = false;  // force re-blur with fresh capture
@@ -241,7 +241,7 @@ void AcrylicMaterial::captureLiveFramebuffer()
     if (!initialized_ || shouldSkipEffect()) return;
     if (viewportWidth_ <= 0 || viewportHeight_ <= 0 || !captureBuffer_.isValid()) return;
     glDisable(GL_SCISSOR_TEST);
-    captureBuffer_.captureScreen(viewportWidth_, viewportHeight_);
+    captureBuffer_.captureScreen(captureSrcWidth(), captureSrcHeight());
     hasValidCapture_ = true;
     blurCacheValid_ = false;  // force re-blur with the fresh live capture
 }
diff --git a/src/ui/effects/acrylic.h b/src/ui/effects/acrylic.h
index 5ef06ec..a21d629 100644
--- a/src/ui/effects/acrylic.h
+++ b/src/ui/effects/acrylic.h
@@ -147,6 +147,13 @@ public:
      * @param height Viewport height
      */
     void resize(int width, int height);
+
+    /// Size of FB 0 in pixels (differs from the logical viewport on HiDPI GL). Used only as the capture
+    /// blit source. Kept apart from viewportWidth_/Height_ so setQuality()/syncBufferLifetime() resets
+    /// of those never lose it. DX11 ignores it (CopyResource needs identical dimensions, scale is 1).
+    void setFramebufferSize(int w, int h) {
+        if (w != fbWidth_ || h != fbHeight_) { fbWidth_ = w; fbHeight_ = h; markBackgroundDirty(); }
+    }
     
     /**
      * @brief Capture current screen content for blur source
@@ -375,6 +382,10 @@ private:
     // Screen dimensions
     int viewportWidth_ = 0;
     int viewportHeight_ = 0;
+    int fbWidth_ = 0;     // FB 0 pixel size (0 = unknown -> use the viewport size)
+    int fbHeight_ = 0;
+    int captureSrcWidth()  const { return fbWidth_  > 0 ? fbWidth_  : viewportWidth_;  }
+    int captureSrcHeight() const { return fbHeight_ > 0 ? fbHeight_ : viewportHeight_; }
     
     // Noise texture config
     int noiseTextureSize_ = 256;  // Texture resolution (larger = less repetition)
diff --git a/src/ui/effects/capture_extent.h b/src/ui/effects/capture_extent.h
new file mode 100644
index 0000000..abf4c44
--- /dev/null
+++ b/src/ui/effects/capture_extent.h
@@ -0,0 +1,12 @@
+#pragma once
+#include <cmath>
+namespace dragonx { namespace ui { namespace effects {
+/// Framebuffer pixel extent for a window-unit extent and ImGui's DisplayFramebufferScale. Same truncation as
+/// imgui_impl_opengl3.cpp (`(int)(DisplaySize.x * DisplayFramebufferScale.x)`), so the blit source matches FB 0.
+inline int framebufferExtent(float logical, float scale) {
+    if (!(scale > 0.0f) || !(logical > 0.0f)) return (int)logical;
+    return (int)(logical * scale);
+}
+/// Source extent for the capture blit: the framebuffer size when known, else the logical size.
+inline int captureSourceExtent(int fbExtent, int logicalExtent) { return fbExtent > 0 ? fbExtent : logicalExtent; }
+}}}
diff --git a/src/ui/effects/framebuffer.cpp b/src/ui/effects/framebuffer.cpp
index 7eb5201..d2920bc 100644
--- a/src/ui/effects/framebuffer.cpp
+++ b/src/ui/effects/framebuffer.cpp
@@ -198,14 +198,14 @@ void Framebuffer::blitFrom(GLuint srcFbo, int srcX, int srcY, int srcWidth, int
     glBindFramebuffer(GL_FRAMEBUFFER, 0);
 }
 
-void Framebuffer::captureScreen(int viewportWidth, int viewportHeight)
+void Framebuffer::captureScreen(int srcWidth, int srcHeight)
 {
     // Blit from default framebuffer (0) to this framebuffer
     glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
     glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_);
     
     glBlitFramebuffer(
-        0, 0, viewportWidth, viewportHeight,   // Source (screen)
+        0, 0, srcWidth, srcHeight,   // Source (screen)
         0, 0, width_, height_,                  // Dest (this FBO)
         GL_COLOR_BUFFER_BIT,
         GL_LINEAR
diff --git a/src/ui/effects/framebuffer.h b/src/ui/effects/framebuffer.h
index c8610d9..5755dfa 100644
--- a/src/ui/effects/framebuffer.h
+++ b/src/ui/effects/framebuffer.h
@@ -117,7 +117,7 @@ public:
      * @param viewportWidth Current viewport width
      * @param viewportHeight Current viewport height
      */
-    void captureScreen(int viewportWidth, int viewportHeight);
+    void captureScreen(int srcWidth, int srcHeight);
 
 private:
     void cleanup();
diff --git a/src/ui/effects/imgui_acrylic.cpp b/src/ui/effects/imgui_acrylic.cpp
index 65a2712..5c4b2e2 100644
--- a/src/ui/effects/imgui_acrylic.cpp
+++ b/src/ui/effects/imgui_acrylic.cpp
@@ -125,6 +125,8 @@ namespace ImGuiAcrylic {
 static bool s_initialized = false;
 static int s_viewportWidth = 0;
 static int s_viewportHeight = 0;
+static int s_fbWidth = 0;
+static int s_fbHeight = 0;
 static bool s_backgroundCaptured = false;
 
 // Track window state for acrylic rendering
@@ -179,11 +181,24 @@ bool IsAvailable()
 // ============================================================================
 
 void BeginFrame(int width, int height)
+{
+    BeginFrame(width, height, width, height);
+}
+
+void BeginFrame(int width, int height, int fbWidth, int fbHeight)
 {
     if (!s_initialized) {
         return;
     }
 
+    // Moving the window between a Retina and a non-Retina display changes the framebuffer size while the
+    // logical size stays put: the capture must be redone against the new pixel grid.
+    if (fbWidth != s_fbWidth || fbHeight != s_fbHeight) {
+        s_fbWidth = fbWidth;
+        s_fbHeight = fbHeight;
+        getAcrylicMaterial().setFramebufferSize(fbWidth, fbHeight);
+    }
+
     // Update viewport dimensions if changed
     if (width != s_viewportWidth || height != s_viewportHeight) {
         s_viewportWidth = width;
@@ -670,6 +685,7 @@ void Shutdown() {}
 bool IsAvailable() { return false; }
 
 void BeginFrame(int, int) {}
+void BeginFrame(int, int, int, int) {}
 void CaptureBackground() {}
 void InvalidateCapture() {}
 void ReleaseIdleBuffers() {}
diff --git a/src/ui/effects/imgui_acrylic.h b/src/ui/effects/imgui_acrylic.h
index cac9be2..caff102 100644
--- a/src/ui/effects/imgui_acrylic.h
+++ b/src/ui/effects/imgui_acrylic.h
@@ -61,7 +61,10 @@ bool IsAvailable();
  * @param width Viewport width
  * @param height Viewport height
  */
-void BeginFrame(int width, int height);
+void BeginFrame(int width, int height);                       ///< framebuffer == window size
+/// HiDPI: fbWidth/fbHeight are FB 0's pixel size (DisplaySize * DisplayFramebufferScale). The capture
+/// and blur targets stay logical-size; only the capture blit source rectangle uses the pixel size.
+void BeginFrame(int width, int height, int fbWidth, int fbHeight);
 
 /**
  * @brief End the acrylic frame
diff --git a/tests/test_phase4.cpp b/tests/test_phase4.cpp
index 822a943..5b053c4 100644
--- a/tests/test_phase4.cpp
+++ b/tests/test_phase4.cpp
@@ -71,6 +71,7 @@
 #include "ui/staleness_badge.h"
 #include "ui/effects/effect_clock.h"
 #include "ui/effects/border_sampling.h"
+#include "ui/effects/capture_extent.h"
 #include "ui/material/rect_cull.h"
 #include "ui/notifications.h"
 #include "data/seed_migration_resume.h"
@@ -17931,6 +17932,17 @@ void testLoadLanguageFromLooseFiles()
     EXPECT_EQ(std::string(i18n.translate("overview")), std::string("Overview"));
 }
 
+void testCaptureExtent()
+{
+    using namespace dragonx::ui::effects;
+    EXPECT_EQ(framebufferExtent(1280.0f, 1.0f), 1280);
+    EXPECT_EQ(framebufferExtent(1280.0f, 2.0f), 2560);
+    EXPECT_EQ(framebufferExtent(1001.0f, 1.5f), 1501);  // truncation, same as imgui_impl_opengl3.cpp
+    EXPECT_EQ(framebufferExtent(800.0f, 0.0f), 800);    // unknown/zero scale -> logical
+    EXPECT_EQ(captureSourceExtent(0, 1280), 1280);
+    EXPECT_EQ(captureSourceExtent(2560, 1280), 2560);
+}
+
 // resources::writeFileIfDifferent (bundled-theme extraction): rewrite a file only when its bytes differ.
 void testWriteFileIfDifferent()
 {
@@ -18312,6 +18324,7 @@ int main()
     testTextFitBanner();
     testLoadLanguageFromLooseFiles();
     testWriteFileIfDifferent();
+    testCaptureExtent();
 
     if (g_failures != 0) {
         std::cerr << g_failures << " test assertion(s) failed\n";