feat: snap threshold is screen-space constant under zoom

SnapParams::for_zoom scales the virtual-space threshold by 1/zoom so
the felt grab distance stays the configured screen size at any zoom
(zoomed out it previously shrank with the desktop, making snapping
fiddly exactly when windows are being arranged from above). Capped at
45% of the cell so a deep zoom-out can't snap from half a period away.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/snap.rs | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/snap.rs b/src/snap.rs
index 8be9fe9..fc0587c 100644
--- a/src/snap.rs
+++ b/src/snap.rs
@@ -46,6 +46,19 @@ pub struct SnapParams {
 }
 
 impl SnapParams {
+    /// Snapping is aimed in SCREEN space: the configured threshold is the
+    /// grab distance at zoom 1, and zooming out must not shrink the felt
+    /// target — so the virtual-space threshold grows by 1/zoom. Capped at
+    /// 45% of the cell so a deep zoom-out can't snap from half a cell away
+    /// (targets are one period apart; past the midpoint snapping would
+    /// thrash between neighbors).
+    pub fn for_zoom(mut self, zoom: f64) -> Self {
+        if self.threshold > 0.0 && zoom > 0.0 && zoom.is_finite() {
+            self.threshold = (self.threshold / zoom).min(self.cell_size * 0.45);
+        }
+        self
+    }
+
     fn enabled(&self) -> bool {
         self.threshold > 0.0 && self.cell_size > 0.5
     }
@@ -233,6 +246,22 @@ mod tests {
         assert_eq!(maximized_span(150.0, 220.0, 100.0, 10.0, 5.0), (115.0, 205.0));
     }
 
+    #[test]
+    fn zoomed_out_threshold_holds_screen_size() {
+        // Threshold 24 at zoom 0.5 → 48 virtual = the same 24 screen px.
+        let p = params().for_zoom(0.5);
+        assert_eq!(p.threshold, 48.0);
+        // Deep zoom-out caps at 45% of the cell (512 → 230.4).
+        let p = params().for_zoom(0.05);
+        assert!((p.threshold - 230.4).abs() < 1e-9);
+        // Zoom 1 unchanged; zoomed in shrinks (still 24 screen px).
+        assert_eq!(params().for_zoom(1.0).threshold, 24.0);
+        assert_eq!(params().for_zoom(2.0).threshold, 12.0);
+        // Disabled stays disabled.
+        let p = SnapParams { threshold: 0.0, ..params() }.for_zoom(0.5);
+        assert_eq!(p.threshold, 0.0);
+    }
+
     #[test]
     fn zero_threshold_disables() {
         let p = SnapParams { threshold: 0.0, ..params() };