wm: keep windows in place when the console scrolls beneath them

The console's batched scroll (gop::shift_up_n) shifts the whole shared
shadow, which dragged any composited window pixels up with it.  Add a
scroll hook so the window manager re-composites every visible window
after a console scroll, restoring them at their fixed positions.

1 files changed, 19 insertions, 0 deletions

diff --git a/rsnix/src/gop.rs b/rsnix/src/gop.rs
index deff226..cddad22 100644
--- a/rsnix/src/gop.rs
+++ b/rsnix/src/gop.rs
@@ -104,6 +104,25 @@ pub fn qr_locked() -> bool {
     QR_LOCKED.load(Ordering::Relaxed)
 }
 
+/// Hook invoked after the console scrolls, so the window manager can
+/// re-composite GUI windows (which sit above the console layer in the
+/// shadow and must not scroll with it).
+fn scroll_hook_noop() {}
+
+static mut SCROLL_HOOK: fn() = scroll_hook_noop;
+
+pub fn set_scroll_hook(f: fn()) {
+    unsafe {
+        SCROLL_HOOK = f;
+    }
+}
+
+pub(crate) fn call_scroll_hook() {
+    unsafe {
+        SCROLL_HOOK();
+    }
+}
+
 #[inline]
 fn stall_display() {
     let n = DISPLAY_STALL.load(Ordering::Relaxed);