Fix a maximized X11 window overhanging the screen by its own border

Reported by the aegis-fc peer session testing srdwm's own layer-shell
strut handling: a maximized X11 client sat 4-8px past the right and
bottom screen edges whenever its border was nonzero.

set_border_width sets the frame's native X11 border-width attribute,
which the X server draws outside a window's own declared width/height on
all four sides - unlike every other backend's own border in this
compositor (rendered as ordinary pixels inside the allocated geometry
rect). apply_geometry configured the frame at geometry's own x/y/width/
height verbatim, so a nonzero native border pushed the frame's true
visible footprint 2*border_width past every edge of what geometry
actually promised.

Fixed by shifting the configured origin inward and the configured size
down by border_width on both axes (frame_geometry_for, pulled out as a
pure function so it's unit-tested without a real X11 connection) - the
visible footprint, native border included, now lands exactly on
geometry. border_width == 0 reduces to the prior behavior exactly.

1 files changed, 32 insertions, 0 deletions

diff --git a/crates/x11/src/platform/tests.rs b/crates/x11/src/platform/tests.rs
index 7af5455..fbae5ff 100644
--- a/crates/x11/src/platform/tests.rs
+++ b/crates/x11/src/platform/tests.rs
@@ -95,3 +95,35 @@
         let usable = usable_rect(full, (1920, 1080), std::iter::empty());
         assert_eq!(usable, full);
     }
+
+    #[test]
+    fn zero_border_width_leaves_the_frame_geometry_unchanged() {
+        let geom = Rect::new(0, 0, 1920, 1080);
+        assert_eq!(frame_geometry_for(geom, 0), (0, 0, 1920, 1080));
+    }
+
+    #[test]
+    fn a_real_border_width_keeps_the_true_footprint_equal_to_the_requested_geometry() {
+        // Live-reported bug: a maximized window with a native X11 border
+        // sat 2*border_width pixels past the right/bottom edges, since the
+        // border draws outside the configured width/height. The true
+        // on-screen footprint - configured origin minus the border on the
+        // near side, configured size plus the border on both sides - must
+        // reproduce the original geometry exactly.
+        let geom = Rect::new(100, 50, 1920, 1080);
+        let (x, y, w, h) = frame_geometry_for(geom, 4);
+        assert_eq!((x - 4, y - 4, w + 8, h + 8), (geom.x, geom.y, geom.width, geom.height));
+    }
+
+    #[test]
+    fn a_maximized_window_no_longer_overhangs_the_monitor_with_a_real_border() {
+        // The exact live scenario: a window maximized to fill the whole
+        // monitor must not visually extend past it just because it also
+        // has a nonzero border.
+        let monitor = Rect::new(0, 0, 1920, 1080);
+        let (x, y, w, h) = frame_geometry_for(monitor, 4);
+        assert_eq!(x - 4, 0, "left edge (including border) must not sit left of the monitor");
+        assert_eq!(y - 4, 0, "top edge (including border) must not sit above the monitor");
+        assert_eq!(x + w as i32 + 4, 1920, "right edge (including border) must not overhang the monitor");
+        assert_eq!(y + h as i32 + 4, 1080, "bottom edge (including border) must not overhang the monitor");
+    }