wm: fix usize underflow in move_window at screen edges

ox.saturating_sub(pad) / oy.saturating_sub(pad) — plain subtraction
on usize wraps to usize::MAX when the window sits at x=0 or y=0,
causing composite_rect to draw at the far end of the framebuffer
instead of near the window.  Also broke drag-to-move when stuck
against the top or left edge.

1 files changed, 4 insertions, 3 deletions

diff --git a/rsnix/src/wm.rs b/rsnix/src/wm.rs
index e5ae6ba..39ad1be 100644
--- a/rsnix/src/wm.rs
+++ b/rsnix/src/wm.rs
@@ -604,9 +604,10 @@ fn move_window(id: usize, nx: usize, ny: usize) {
         WINDOWS[id].x = nx;
         WINDOWS[id].y = ny;
     }
-    let pad = 1;
-    composite_rect(ox - pad, oy - pad, w.w + 2 * pad, w.h + TITLE_BAR_H + 2 * pad);
-    composite_rect(nx - pad, ny - pad, w.w + 2 * pad, w.h + TITLE_BAR_H + 2 * pad);
+    let pad: usize = 1;
+    let oh = w.h + TITLE_BAR_H;
+    composite_rect(ox.saturating_sub(pad), oy.saturating_sub(pad), w.w + 2 * pad, oh + 2 * pad);
+    composite_rect(nx.saturating_sub(pad), ny.saturating_sub(pad), w.w + 2 * pad, oh + 2 * pad);
 }
 
 pub fn owner(id: usize) -> Option<usize> {