Scale text by font size ratio when applying system theme

desktop/src/main.rs

@@ -37,7 +37,14 @@ fn main() {
             }
 
             match SystemTheme::from_system() {
-                Ok(system) => apply_system_theme(&system, cx),
+                Ok(mut system) => {
+                    let native = system.light.defaults.font.size;
+                    if native > 0.0 {
+                        let scaling = settings.theme.font_size / native;
+                        system.accessibility.text_scaling_factor *= scaling;
+                    }
+                    apply_system_theme(&system, cx);
+                }
                 Err(error) => {
                     tracing::warn!("native theme unavailable ({error})");
 

docs/NATIVE-THEME-MIGRATION.md

@@ -147,6 +147,15 @@ match native_theme_gpui::SystemTheme::from_system() {
 configs, so the subsequent `Theme::change` / `sync_system_appearance` for the
 user's `AppearanceMode` picks the right native palette.
 
+Sizing note: gpui-component derives the window rem from `theme.font_size`, so
+every rem-relative element scales with it. The OS-reported UI font (13 on
+macOS) is smaller than the 16 this app is designed around, so the native path
+multiplies `system.accessibility.text_scaling_factor` by
+`settings.theme.font_size / native_size` before applying — the connector's
+supported scaling knob, which also preserves any OS accessibility scaling.
+The same factor scales `mono_font_size`, so code text follows the UI size
+(the pre-migration app used 13 for mono while the UI was 16).
+
 On the native path, do **not** set `radius` / `font_size` / `light_theme` /
 `dark_theme` etc. on `Theme::global_mut` — the connector owns those fields,
 and overwriting them fights its observer.