feat(options): interface scale for the whole window

On a non-Retina screen macOS draws the interface at 1x, which reads small
on a Full HD monitor, and there was no way to enlarge it. Options >
Display now offers an interface scale of 75–200 % in 25 % steps, applied
through the window's scale factor on top of the display's own and kept
with the user settings. It is a list rather than a slider because the
dialog rescales under the pointer as the value changes.

Fixes #1655

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -561,6 +561,8 @@ pub(super) struct OpenCADStudio {
     zoom_wheel_reversed: bool,
     /// Scroll pans and Ctrl+scroll zooms (touchpads); see `scroll_intent`.
     touchpad_pan: bool,
+    /// Interface scale in percent, applied through the window scale factor.
+    ui_scale_percent: u16,
     /// Mouse-wheel zoom sensitivity (ZOOMFACTOR), held inside
     /// `settings::ZOOM_FACTOR_MIN..=settings::ZOOM_FACTOR_MAX`. `SETVAR`
     /// only sets the range the system variable itself has; the Options
@@ -2378,6 +2380,8 @@ pub enum Message {
     /// Toggle reversing the mouse-wheel zoom direction (ZOOMWHEEL).
     ZoomWheelReversedChanged(bool),
     TouchpadPanChanged(bool),
+    /// Interface scale in percent (Options > Display).
+    UiScaleChanged(u16),
     /// Change how far one wheel notch zooms, from the Options slider
     /// (ZOOMFACTOR).
     ZoomFactorChanged(i32),
@@ -4161,6 +4165,7 @@ impl OpenCADStudio {
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
             touchpad_pan: false,
+            ui_scale_percent: 100,
             zoom_factor: 60,
             zoom_factor_input: 60.to_string(),
             cursor_size: 5,
@@ -4904,6 +4909,7 @@ pub fn run() -> iced::Result {
         ..iced::Settings::default()
     })
     .subscription(OpenCADStudio::subscription)
+    .scale_factor(|state: &OpenCADStudio, _| f32::from(state.ui_scale_percent) / 100.0)
     .title(|state: &OpenCADStudio, window_id: window::Id| {
         let _ = window_id; // all dialogs are in-canvas modals now
         if let Some(tab) = state.tabs.get(state.active_tab) {

src/app/settings.rs

@@ -183,6 +183,11 @@ pub const ZOOM_FACTOR_MAX: i32 = 500;
 /// Hold a ZOOMFACTOR inside the range the application can store, wherever it
 /// arrives from — the slider, the Options field, or a configuration file
 /// written by hand or by an older build.
+/// Interface scale in percent, kept to the range the Options slider offers.
+pub fn clamp_ui_scale(percent: u16) -> u16 {
+    percent.clamp(75, 200)
+}
+
 pub fn clamp_zoom_factor(v: i32) -> i32 {
     v.clamp(ZOOM_FACTOR_MIN, ZOOM_FACTOR_MAX)
 }
@@ -391,6 +396,8 @@ pub struct UserSettings {
     /// Scrolling pans and Ctrl+scroll (a touchpad pinch) zooms, for a
     /// touchpad that reports two-finger drags as wheel scrolls.
     pub touchpad_pan: bool,
+    /// Interface scale in percent, on top of the display's own scale.
+    pub ui_scale_percent: u16,
     pub zoom_factor: i32,
     /// CURSORSIZE: normalized crosshair reach; 5 retains the original 60 px arms.
     pub cursor_size: i32,
@@ -811,6 +818,7 @@ impl Default for UserSettings {
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
             touchpad_pan: false,
+            ui_scale_percent: 100,
             zoom_factor: 60,
             cursor_size: 5,
             pick_box: 3,

src/app/update/file.rs

@@ -663,6 +663,7 @@ impl OpenCADStudio {
             polar_increment_deg: self.polar_increment_deg,
             zoom_wheel_reversed: self.zoom_wheel_reversed,
             touchpad_pan: self.touchpad_pan,
+            ui_scale_percent: self.ui_scale_percent,
             zoom_factor: self.zoom_factor,
             cursor_size: self.cursor_size,
             pick_box: self.pick_box,
@@ -764,6 +765,7 @@ impl OpenCADStudio {
         self.polar_increment_deg = s.polar_increment_deg;
         self.zoom_wheel_reversed = s.zoom_wheel_reversed;
         self.touchpad_pan = s.touchpad_pan;
+        self.ui_scale_percent = crate::app::settings::clamp_ui_scale(s.ui_scale_percent);
         self.zoom_factor = crate::app::settings::clamp_zoom_factor(s.zoom_factor);
         // Like the drafting-rotation field below, the Options field edits a
         // buffer rather than the value, so it is reseeded whenever the value

src/app/update/mod.rs

@@ -8391,6 +8391,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::UiScaleChanged(percent) => {
+                self.ui_scale_percent = crate::app::settings::clamp_ui_scale(percent);
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
             Message::ZoomFactorChanged(factor) => {
                 self.zoom_factor = crate::app::settings::clamp_zoom_factor(factor);
                 // The field beside the slider reads the same value, so the

src/app/view/modal.rs

@@ -317,6 +317,7 @@ impl OpenCADStudio {
                             commandline_fade_ms: self.commandline_fade_ms,
                             zoom_wheel_reversed: self.zoom_wheel_reversed,
                             touchpad_pan: self.touchpad_pan,
+                            ui_scale_percent: self.ui_scale_percent,
                             zoom_factor: self.zoom_factor,
                             texteditmode: self.texteditmode,
                             dimension_continue_mode: self.dimension_continue_mode,

src/locale_catalog.rs

@@ -4798,6 +4798,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Zoom" => Some(("common", "zoom")),
         "Reverse mouse wheel zoom (ZOOMWHEEL)" => Some(("common", "reverse-mouse-wheel-zoom-zoomwheel")),
         "Touchpad: scroll pans, Ctrl+scroll or pinch zooms" => Some(("common", "touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms")),
+        "Interface scale" => Some(("common", "interface-scale")),
         "Zoom factor" => Some(("common", "zoom-factor")),
         "How far one wheel notch zooms (ZOOMFACTOR)." => Some(("common", "how-far-one-wheel-notch-zooms-zoomfactor")),
         "Text and Dimensions" => Some(("common", "text-and-dimensions")),

src/ui/window/options.rs

@@ -61,6 +61,8 @@ pub struct AppPrefs {
     pub zoom_wheel_reversed: bool,
     /// Scroll pans and Ctrl+scroll zooms (touchpads).
     pub touchpad_pan: bool,
+    /// Interface scale in percent.
+    pub ui_scale_percent: u16,
     /// ZOOMFACTOR. The slider sets the range the system variable has; the
     /// field beside it reaches `settings::ZOOM_FACTOR_MAX`.
     pub zoom_factor: i32,
@@ -679,6 +681,23 @@ pub fn view_window<'a>(
         .width(sizing.width),
         Space::new().height(12),
         color_controls,
+        Space::new().height(16),
+        // A list, not a slider: the dialog rescales under the cursor as the
+        // value changes, which would drag a slider along with it.
+        row![
+            text(crate::t!("Interface scale")).size(12).width(150),
+            iced::widget::pick_list(
+                Some(format!("{}%", prefs.ui_scale_percent)),
+                [75u16, 100, 125, 150, 175, 200].map(|p| format!("{p}%")).to_vec(),
+                |label: &String| label.clone(),
+            )
+            .on_select(|label: String| {
+                Message::UiScaleChanged(label.trim_end_matches('%').parse().unwrap_or(100))
+            })
+            .width(sizing.width),
+        ]
+        .spacing(12)
+        .align_y(iced::Center),
         Space::new().height(24),
         row![
             text(crate::t!("Model Space Appearance")).size(15),