diff --git a/api/cpp/cbindgen.rs b/api/cpp/cbindgen.rs
index f80f268c0..7a05dee04 100644
--- a/api/cpp/cbindgen.rs
+++ b/api/cpp/cbindgen.rs
@@ -563,7 +563,7 @@ fn gen_corelib(
             "slint_windowrc_supports_native_menu_bar",
             "slint_windowrc_setup_native_menu_bar",
             "slint_windowrc_show_native_popup_menu",
-            "slint_windowrc_default_font_size",
+            "slint_windowrc_resolved_default_font_size",
             "slint_windowrc_dispatch_pointer_event",
             "slint_windowrc_dispatch_key_event",
             "slint_windowrc_dispatch_event",
diff --git a/api/cpp/include/slint_window.h b/api/cpp/include/slint_window.h
index 64d70cffa..890d1fcda 100644
--- a/api/cpp/include/slint_window.h
+++ b/api/cpp/include/slint_window.h
@@ -230,9 +230,9 @@ public:
         cbindgen_private::slint_register_bitmap_font(&inner, &font);
     }
 
-    inline float default_font_size() const
+    inline float resolved_default_font_size() const
     {
-        return cbindgen_private::slint_windowrc_default_font_size(&inner);
+        return cbindgen_private::slint_windowrc_resolved_default_font_size(&inner);
     }
 
     /// \private
diff --git a/internal/backends/qt/qt_window.rs b/internal/backends/qt/qt_window.rs
index c723876a6..6c69d8f52 100644
--- a/internal/backends/qt/qt_window.rs
+++ b/internal/backends/qt/qt_window.rs
@@ -2400,6 +2400,16 @@ impl i_slint_core::renderer::RendererSealed for QtWindow {
         Ok(())
     }
 
+    fn default_font_size(&self) -> LogicalLength {
+        let default_font_size = cpp!(unsafe[] -> i32 as "int" {
+            return QFontInfo(qApp->font()).pixelSize();
+        });
+        // Ideally this would return the value from another property with a binding that's updated
+        // as a FontChange event is received. This is relevant for the case of using the Qt backend
+        // with a non-native style.
+        LogicalLength::new(default_font_size as f32)
+    }
+
     fn free_graphics_resources(
         &self,
         component: ItemTreeRef,
diff --git a/internal/backends/testing/testing_backend.rs b/internal/backends/testing/testing_backend.rs
index e8cdcc407..85bbdcce4 100644
--- a/internal/backends/testing/testing_backend.rs
+++ b/internal/backends/testing/testing_backend.rs
@@ -235,6 +235,10 @@ impl RendererSealed for TestingWindow {
         Ok(())
     }
 
+    fn default_font_size(&self) -> LogicalLength {
+        LogicalLength::new(10.)
+    }
+
     fn set_window_adapter(&self, _window_adapter: &Rc<dyn WindowAdapter>) {
         // No-op since TestingWindow is also the WindowAdapter
     }
diff --git a/internal/compiler/generator/cpp.rs b/internal/compiler/generator/cpp.rs
index 01d2502f1..a97ede07b 100644
--- a/internal/compiler/generator/cpp.rs
+++ b/internal/compiler/generator/cpp.rs
@@ -3560,7 +3560,7 @@ fn compile_builtin_function_call(
             format!("{}.scale_factor()", access_window_field(ctx))
         }
         BuiltinFunction::GetWindowDefaultFontSize => {
-            format!("{}.default_font_size()", access_window_field(ctx))
+            format!("{}.resolved_default_font_size()", access_window_field(ctx))
         }
         BuiltinFunction::AnimationTick => "slint::cbindgen_private::slint_animation_tick()".into(),
         BuiltinFunction::Debug => {
diff --git a/internal/compiler/generator/rust.rs b/internal/compiler/generator/rust.rs
index b58cc02c9..b1b382c69 100644
--- a/internal/compiler/generator/rust.rs
+++ b/internal/compiler/generator/rust.rs
@@ -3022,7 +3022,7 @@ fn compile_builtin_function_call(
         }
         BuiltinFunction::GetWindowDefaultFontSize => {
             let window_adapter_tokens = access_window_adapter_field(ctx);
-            quote!(sp::WindowInner::from_pub(#window_adapter_tokens.window()).window_item().unwrap().as_pin_ref().default_font_size().get())
+            quote!(sp::WindowItem::resolved_default_font_size(&sp::WindowInner::from_pub(#window_adapter_tokens.window()).window_item_rc().unwrap()).get())
         }
         BuiltinFunction::AnimationTick => {
             quote!(sp::animation_tick())
diff --git a/internal/core/items.rs b/internal/core/items.rs
index 161384be1..e671baf94 100644
--- a/internal/core/items.rs
+++ b/internal/core/items.rs
@@ -1325,6 +1325,11 @@ impl WindowItem {
         }
     }
 
+    pub fn resolved_default_font_size(self_rc: &ItemRc) -> LogicalLength {
+        Self::resolve_font_property(&self_rc, Self::font_size)
+            .unwrap_or_else(|| self_rc.window_adapter().unwrap().renderer().default_font_size())
+    }
+
     fn resolve_font_property<T>(
         self_rc: &ItemRc,
         property_fn: impl Fn(Pin<&Self>) -> Option<T>,
diff --git a/internal/core/renderer.rs b/internal/core/renderer.rs
index bd82fb0c4..8924fee0d 100644
--- a/internal/core/renderer.rs
+++ b/internal/core/renderer.rs
@@ -118,6 +118,8 @@ pub trait RendererSealed {
 
     fn set_window_adapter(&self, _window_adapter: &Rc<dyn WindowAdapter>);
 
+    fn default_font_size(&self) -> LogicalLength;
+
     fn resize(&self, _size: crate::api::PhysicalSize) -> Result<(), PlatformError> {
         Ok(())
     }
diff --git a/internal/core/software_renderer.rs b/internal/core/software_renderer.rs
index 735270a62..50be18c18 100644
--- a/internal/core/software_renderer.rs
+++ b/internal/core/software_renderer.rs
@@ -883,6 +883,10 @@ impl RendererSealed for SoftwareRenderer {
         self::fonts::systemfonts::register_font_from_path(path)
     }
 
+    fn default_font_size(&self) -> LogicalLength {
+        self::fonts::DEFAULT_FONT_SIZE
+    }
+
     fn set_window_adapter(&self, window_adapter: &Rc<dyn WindowAdapter>) {
         *self.maybe_window_adapter.borrow_mut() = Some(Rc::downgrade(window_adapter));
         self.partial_rendering_state.clear_cache();
diff --git a/internal/core/window.rs b/internal/core/window.rs
index cb43a5d92..643e4c8e3 100644
--- a/internal/core/window.rs
+++ b/internal/core/window.rs
@@ -1521,6 +1521,7 @@ pub mod ffi {
     use crate::api::{RenderingNotifier, RenderingState, SetRenderingNotifierError};
     use crate::graphics::Size;
     use crate::graphics::{IntSize, Rgba8Pixel};
+    use crate::items::WindowItem;
     use crate::SharedVector;
 
     /// This enum describes a low-level access to specific graphics APIs used
@@ -1900,11 +1901,12 @@ pub mod ffi {
 
     /// Return the default-font-size property of the WindowItem
     #[unsafe(no_mangle)]
-    pub unsafe extern "C" fn slint_windowrc_default_font_size(
+    pub unsafe extern "C" fn slint_windowrc_resolved_default_font_size(
         handle: *const WindowAdapterRcOpaque,
     ) -> f32 {
         let window_adapter = &*(handle as *const Rc<dyn WindowAdapter>);
-        window_adapter.window().0.window_item().unwrap().as_pin_ref().default_font_size().get()
+        WindowItem::resolved_default_font_size(&window_adapter.window().0.window_item_rc().unwrap())
+            .get()
     }
 
     /// Dispatch a key pressed or release event
diff --git a/internal/interpreter/eval.rs b/internal/interpreter/eval.rs
index 043430218..b3fe7085b 100644
--- a/internal/interpreter/eval.rs
+++ b/internal/interpreter/eval.rs
@@ -22,7 +22,7 @@ use i_slint_compiler::namedreference::NamedReference;
 use i_slint_compiler::object_tree::ElementRc;
 use i_slint_core as corelib;
 use i_slint_core::input::FocusReason;
-use i_slint_core::items::ItemRc;
+use i_slint_core::items::{ItemRc, WindowItem};
 use smol_str::SmolStr;
 use std::collections::HashMap;
 use std::rc::Rc;
@@ -432,7 +432,7 @@ fn call_builtin_function(
         ),
         BuiltinFunction::GetWindowDefaultFontSize => {
             Value::Number(local_context.component_instance.access_window(|window| {
-                window.window_item().unwrap().as_pin_ref().default_font_size().get()
+                WindowItem::resolved_default_font_size(&window.window_item_rc().unwrap()).get()
             }) as _)
         }
         BuiltinFunction::AnimationTick => {
diff --git a/internal/renderers/femtovg/lib.rs b/internal/renderers/femtovg/lib.rs
index 59b318ebb..fc57c05e0 100644
--- a/internal/renderers/femtovg/lib.rs
+++ b/internal/renderers/femtovg/lib.rs
@@ -428,6 +428,10 @@ impl<B: GraphicsBackend> RendererSealed for FemtoVGRenderer<B> {
         sharedfontdb::register_font_from_path(path)
     }
 
+    fn default_font_size(&self) -> LogicalLength {
+        self::fonts::DEFAULT_FONT_SIZE
+    }
+
     fn set_rendering_notifier(
         &self,
         callback: Box<dyn i_slint_core::api::RenderingNotifier>,
diff --git a/internal/renderers/skia/lib.rs b/internal/renderers/skia/lib.rs
index 245410672..d2f57ecb7 100644
--- a/internal/renderers/skia/lib.rs
+++ b/internal/renderers/skia/lib.rs
@@ -899,6 +899,10 @@ impl i_slint_core::renderer::RendererSealed for SkiaRenderer {
         }
     }
 
+    fn default_font_size(&self) -> LogicalLength {
+        self::textlayout::DEFAULT_FONT_SIZE
+    }
+
     fn free_graphics_resources(
         &self,
         component: i_slint_core::item_tree::ItemTreeRef,
diff --git a/tests/cases/types/rem.slint b/tests/cases/types/rem.slint
index 28e148332..a6e797c1e 100644
--- a/tests/cases/types/rem.slint
+++ b/tests/cases/types/rem.slint
@@ -1,26 +1,25 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
-global SomeGlobal := {
-    property<relative-font-size> global_rem: 4rem;
+global SomeGlobal {
+    out property <relative-font-size> global_rem: 4rem;
 }
 
-TestCase := Window {
-    default-font-size: 10px;
-    property<length> normal: 1rem;
-    property<length> double: 2rem;
-    property<length> half: 0.5rem;
-    property <relative-font-size> four_rem: 4rem;
-    property<length> four: four_rem;
+export component TestCase inherits Window {
+    out property <length> normal: 1rem;
+    out property <length> double: 2rem;
+    out property <length> half: 0.5rem;
+    out property <relative-font-size> four_rem: 4rem;
+    out property <length> four: four_rem;
 
-    property<physical-length> phys-pixel-size: 1rem;
+    out property <physical-length> phys-pixel-size: 1rem;
 
-    property <relative-font-size> px_to_rem: 20px;
-    property <relative-font-size> phx_to_rem: 20phx;
+    out property <relative-font-size> px_to_rem: 20px;
+    out property <relative-font-size> phx_to_rem: 20phx;
 
-    property <bool> cmp_test: normal < 1rem && double > 1rem;
+    out property <bool> cmp_test: normal < 1rem && double > 1rem;
 
-    property<bool> test:(normal == 10px && double == 20px && half == 5px && four_rem == 40px && SomeGlobal.global_rem == 40px && phys-pixel-size == 10phx && px_to_rem == 2rem);
+    out property <bool> test:(normal == 10px && double == 20px && half == 5px && four_rem == 40px && SomeGlobal.global_rem == 40px && phys-pixel-size == 10phx && px_to_rem == 2rem);
 }
 
 /*