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.

3 files changed, 37 insertions, 7 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);
diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index c5fc5fd..4ef568d 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -458,13 +458,7 @@ fn put_byte(b: u8) {
 }
 
 fn param(i: usize, default: usize) -> usize {
-    unsafe {
-        if i < NPARAMS {
-            PARAMS[i]
-        } else {
-            default
-        }
-    }
+    unsafe { if i < NPARAMS { PARAMS[i] } else { default } }
 }
 
 /// Place a printable glyph at the cursor and advance (wrapping/scrolling).
@@ -694,6 +688,7 @@ pub fn sync_scroll() {
         PENDING_SCROLLS = 0;
         LAST_SYNC_TICK = crate::interrupts::TICKS.load(core::sync::atomic::Ordering::Relaxed);
         flush();
+        gop::call_scroll_hook();
     }
 }
 
diff --git a/rsnix/src/wm.rs b/rsnix/src/wm.rs
index be74f9a..1326696 100644
--- a/rsnix/src/wm.rs
+++ b/rsnix/src/wm.rs
@@ -90,6 +90,8 @@ pub fn init() {
         CASCADE_Y = 0;
     }
     FOCUSED.store(CONSOLE, Ordering::Relaxed);
+    // When the console scrolls, re-composite windows so they stay put.
+    gop::set_scroll_hook(redraw_windows);
 }
 
 /// The focused window, or `None` if the console has focus.
@@ -661,6 +663,20 @@ pub fn spawn_input_pump() -> Option<usize> {
     crate::sched::spawn(pump_main, "wm-input")
 }
 
+/// Re-composite every visible window.  Called after the console scrolls so
+/// windows stay in place while the text layer beneath them shifts.
+pub fn redraw_windows() {
+    unsafe {
+        for i in 0..ZLEN {
+            let id = ZORDER[i];
+            let w = WINDOWS[id];
+            if w.used && w.state == WinState::Visible {
+                composite_rect(w.x, w.y, w.w, w.h + TITLE_BAR_H);
+            }
+        }
+    }
+}
+
 fn pump_main() {
     loop {
         if focused().is_some() {