tree: move container scroll logic to separate util

src/utils/scroller.rs

@@ -0,0 +1,29 @@
+use {
+    crate::ifs::wl_seat::{
+        wl_pointer::{PendingScroll, VERTICAL_SCROLL},
+        PX_PER_SCROLL,
+    },
+    std::cell::Cell,
+};
+
+#[derive(Default)]
+pub struct Scroller {
+    scroll: Cell<f64>,
+}
+
+impl Scroller {
+    pub fn handle(&self, scroll: &PendingScroll) -> Option<i32> {
+        if let Some(d) = scroll.discrete[VERTICAL_SCROLL as usize].get() {
+            self.scroll.set(0.0);
+            Some(d)
+        } else if let Some(scroll) = scroll.axis[VERTICAL_SCROLL as usize].get() {
+            let mut scroll = self.scroll.get() + scroll.to_f64();
+            let discrete = (scroll / PX_PER_SCROLL).trunc();
+            scroll -= discrete * PX_PER_SCROLL;
+            self.scroll.set(scroll);
+            Some(discrete as i32)
+        } else {
+            None
+        }
+    }
+}