Round scale to closest representable

2 files changed, 21 insertions, 3 deletions

diff --git a/src/niri.rs b/src/niri.rs
index b8309f65..13e46b08 100644
--- a/src/niri.rs
+++ b/src/niri.rs
@@ -124,7 +124,7 @@ use crate::ui::exit_confirm_dialog::ExitConfirmDialog;
 use crate::ui::hotkey_overlay::HotkeyOverlay;
 use crate::ui::screen_transition::{self, ScreenTransition};
 use crate::ui::screenshot_ui::{ScreenshotUi, ScreenshotUiRenderElement};
-use crate::utils::scale::guess_monitor_scale;
+use crate::utils::scale::{closest_representable_scale, guess_monitor_scale};
 use crate::utils::spawning::CHILD_ENV;
 use crate::utils::{
     center, center_f64, get_monotonic_time, ipc_transform_to_smithay, logical_output,
@@ -1053,7 +1053,7 @@ impl State {
                 let resolution = output.current_mode().unwrap().size;
                 guess_monitor_scale(size_mm, resolution)
             });
-            let scale = scale.clamp(1., 10.);
+            let scale = closest_representable_scale(scale.clamp(1., 10.));
 
             let mut transform = config
                 .map(|c| ipc_transform_to_smithay(c.transform))
@@ -1736,7 +1736,8 @@ impl Niri {
             let resolution = output.current_mode().unwrap().size;
             guess_monitor_scale(size_mm, resolution)
         });
-        let scale = scale.clamp(1., 10.);
+        let scale = closest_representable_scale(scale.clamp(1., 10.));
+
         let mut transform = c
             .map(|c| ipc_transform_to_smithay(c.transform))
             .unwrap_or(Transform::Normal);
diff --git a/src/utils/scale.rs b/src/utils/scale.rs
index 339d0678..6db88248 100644
--- a/src/utils/scale.rs
+++ b/src/utils/scale.rs
@@ -51,6 +51,13 @@ fn is_valid_for_resolution(resolution: Size<i32, Physical>, scale: i32) -> bool
     logical.w * logical.h >= MIN_LOGICAL_AREA
 }
 
+/// Adjusts the scale to the closest exactly-representable value.
+pub fn closest_representable_scale(scale: f64) -> f64 {
+    // Current fractional-scale Wayland protocol can only represent N / 120 scales.
+    const FRACTIONAL_SCALE_DENOM: f64 = 120.;
+
+    (scale * FRACTIONAL_SCALE_DENOM).round() / FRACTIONAL_SCALE_DENOM
+}
 #[cfg(test)]
 mod tests {
     use k9::snapshot;
@@ -101,4 +108,14 @@ mod tests {
     fn guess_monitor_scale_unknown_size() {
         assert_eq!(check((0, 0), (1920, 1080)), 1.);
     }
+
+    #[test]
+    fn test_round_scale() {
+        snapshot!(closest_representable_scale(1.3), "1.3");
+        snapshot!(closest_representable_scale(1.31), "1.3083333333333333");
+        snapshot!(closest_representable_scale(1.32), "1.3166666666666667");
+        snapshot!(closest_representable_scale(1.33), "1.3333333333333333");
+        snapshot!(closest_representable_scale(1.34), "1.3416666666666666");
+        snapshot!(closest_representable_scale(1.35), "1.35");
+    }
 }