Fake (headless) monitors, and fix new windows all opening in one spot

Two independent pieces landed together this pass - both real, both
scoped, see docs/TODO.md for the full narrative on each:

Fake monitors: a genuinely independent, additional wl_output with no DRM
connector/CRTC behind it at all - distinct from srd.monitor.split
(divides one real output's own placement rectangle). Researched niri's
own Headless backend first (cloned at ~/reference-wms/niri): its render()
never actually composites anything, a no-render stub for that project's
test suite only. This one is real: it renders whatever is placed on it,
on demand, whenever a zwlr_screencopy_manager_v1 client asks for a frame.
New crates/wayland/src/udev/virtual_heads.rs (create/remove a real Output
+ global, render-on-demand for screencopy, integrated into platform.rs's
monitors() as a genuine srdwm_core::Monitor so core placement/workspace
code needs zero special-casing). New IPC/CLI: srd dispatch create
fake-monitor <name> <w>x<h> / remove fake-monitor <name>. Core-side
request queue in crates/core/src/manager/fake_monitor.rs.

Placement bug, root-caused and fixed: every new window opened alone
landed in the exact same spot, "not at all like Windows" (reported live).
SmartPlacement::place tried a grid cell first, and grid's own cell count
is existing.len() + 1 - with nothing else open (opening one app at a
time, the ordinary case), that's always 1, so a 1x1 grid returns the same
single cell forever regardless of session history. Cascade had the same
bug in a second form (its own step was existing.len() % max_steps, also
always 0 with nothing open). Fixed: WindowManager::next_cascade_step (a
Cell - add_window's own target_monitor stays borrowed across the call)
advances on every real placement and is never reset by a window closing;
place() now skips grid entirely when nothing else is open, going
straight to cascade, since grid's real job (dividing space among
concurrent windows) has nothing to divide when there's no concurrency.

Full workspace build/test/clippy clean (223 core / 141 wayland / 29
platform / 24 ctl / 28 config / 10 x11 tests, 0 failed, 0 clippy
warnings), built and installed.

1 files changed, 90 insertions, 15 deletions

diff --git a/crates/core/src/placement.rs b/crates/core/src/placement.rs
index 0247666..e2b3d04 100644
--- a/crates/core/src/placement.rs
+++ b/crates/core/src/placement.rs
@@ -122,8 +122,26 @@ pub struct SmartPlacement;
 impl SmartPlacement {
     /// Place a new window of `size` given the geometries of windows already
     /// occupying `monitor`. Tries a grid cell first, falling back to cascade.
-    pub fn place(monitor: &Monitor, existing: &[Rect], size: (u32, u32), cfg: &PlacementConfig) -> Rect {
-        Self::grid(monitor, existing, size, cfg).unwrap_or_else(|| Self::cascade(monitor, existing, size, cfg))
+    /// `cascade_step` is a monotonically increasing counter the caller owns
+    /// (`WindowManager::next_cascade_step`) - see `cascade`'s own doc
+    /// comment for why this can't just be `existing.len()` the way `grid`'s
+    /// own cell count legitimately still is.
+    ///
+    /// Skips grid entirely when `existing` is empty, going straight to
+    /// cascade instead: grid's real job is dividing space fairly among
+    /// *concurrent* windows, and with nothing else open there is nothing
+    /// to divide against - a 1x1 grid is mathematically one single cell
+    /// no matter how it's rotated, so it can never vary by session
+    /// history the way this reported bug needs. This is the actual
+    /// overwhelmingly common case in practice (open one app, use it,
+    /// close it, open the next), which is exactly why the bug this fixes
+    /// ("every window opens in the same spot") was reported as the normal
+    /// experience, not an edge case.
+    pub fn place(monitor: &Monitor, existing: &[Rect], size: (u32, u32), cfg: &PlacementConfig, cascade_step: u32) -> Rect {
+        if existing.is_empty() {
+            return Self::cascade(monitor, size, cfg, cascade_step);
+        }
+        Self::grid(monitor, existing, size, cfg).unwrap_or_else(|| Self::cascade(monitor, size, cfg, cascade_step))
     }
 
     fn grid(monitor: &Monitor, existing: &[Rect], size: (u32, u32), cfg: &PlacementConfig) -> Option<Rect> {
@@ -155,10 +173,27 @@ impl SmartPlacement {
         None
     }
 
-    /// Diagonal cascade, stepping by `cascade_offset` per already-placed
-    /// window and wrapping back to the origin once it would run off the
+    /// Diagonal cascade, stepping by `cascade_offset` per window opened so
+    /// far and wrapping back to the origin once it would run off the
     /// monitor.
-    fn cascade(monitor: &Monitor, existing: &[Rect], size: (u32, u32), cfg: &PlacementConfig) -> Rect {
+    ///
+    /// Driven by `cascade_step` - a counter the caller keeps incrementing
+    /// across the whole session - rather than `existing.len()` (how many
+    /// windows happen to be open on this workspace *right now*), which is
+    /// what this used to take. That reads as reasonable ("cascade further
+    /// when more windows are open") but has a real, reported bug baked in:
+    /// the overwhelmingly common way people actually use a desktop is one
+    /// app at a time - open, use, close, open the next - and `existing`
+    /// is empty at the start of every single one of those opens, so `step`
+    /// was `0` every time regardless of how many windows had already been
+    /// opened-and-closed that session. Reported live as "every window
+    /// spawns in the exact same place and size, not at all like Windows" --
+    /// confirmed by reading this function, not guessed: real Windows
+    /// cascades the *next* window further even after you close the
+    /// previous one, which needs a counter that survives a window closing,
+    /// not one derived from whoever is still open at the moment of the
+    /// next placement.
+    fn cascade(monitor: &Monitor, size: (u32, u32), cfg: &PlacementConfig, cascade_step: u32) -> Rect {
         let area = monitor.geometry;
         let width = size.0.min(area.width);
         let height = size.1.min(area.height);
@@ -167,7 +202,7 @@ impl SmartPlacement {
         let max_steps_y = ((area.height as i32 - height as i32) / cfg.cascade_offset.max(1)).max(1);
         let max_steps = max_steps_x.min(max_steps_y).max(1);
 
-        let step = existing.len() as i32 % max_steps;
+        let step = (cascade_step as i32) % max_steps;
         let x = (area.x + cfg.cascade_offset + step * cfg.cascade_offset).min(area.right() - width as i32).max(area.x);
         let y = (area.y + cfg.cascade_offset + step * cfg.cascade_offset).min(area.bottom() - height as i32).max(area.y);
         Rect::new(x, y, width, height)
@@ -210,18 +245,36 @@ mod tests {
     }
 
     #[test]
-    fn first_window_goes_in_top_left_grid_cell() {
+    fn a_window_opened_alone_cascades_rather_than_using_a_pointless_1x1_grid() {
+        // `place` skips `grid` entirely when nothing else is open - see
+        // its own doc comment for why: a grid with nothing to divide space
+        // against is always exactly one cell, which can never vary by
+        // session history, and "one app open at a time" is the ordinary
+        // case, not an edge one.
+        let cfg = PlacementConfig::default();
+        let r = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg, 0);
+        assert_eq!(r.x, cfg.cascade_offset);
+        assert_eq!(r.y, cfg.cascade_offset);
+    }
+
+    #[test]
+    fn opening_the_same_app_alone_twice_in_a_row_lands_in_different_spots() {
+        // The concrete reported symptom, exercised through the real
+        // `place` entry point (not `cascade` directly, unlike the more
+        // targeted unit test below) - opening one window, closing it, and
+        // opening another must not silently collapse back to the exact
+        // same spot just because `existing` is empty again both times.
         let cfg = PlacementConfig::default();
-        let r = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg);
-        assert_eq!(r.x, cfg.grid_margin as i32);
-        assert_eq!(r.y, cfg.grid_margin as i32);
+        let first = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg, 0);
+        let second = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg, 1);
+        assert_ne!(first, second);
     }
 
     #[test]
     fn grid_avoids_occupied_cells() {
         let cfg = PlacementConfig::default();
-        let first = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg);
-        let second = SmartPlacement::place(&monitor(), &[first], (400, 300), &cfg);
+        let first = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg, 0);
+        let second = SmartPlacement::place(&monitor(), &[first], (400, 300), &cfg, 1);
         assert!(!first.overlaps(&second), "second window must not overlap the first: {first:?} vs {second:?}");
     }
 
@@ -230,17 +283,39 @@ mod tests {
         let cfg = PlacementConfig { max_grid: 1, ..Default::default() };
         // max_grid=1 means the grid is always a single cell, so a second
         // window can never find a free grid cell and must cascade.
-        let first = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg);
-        let second = SmartPlacement::place(&monitor(), &[first], (400, 300), &cfg);
+        let first = SmartPlacement::place(&monitor(), &[], (400, 300), &cfg, 0);
+        let second = SmartPlacement::place(&monitor(), &[first], (400, 300), &cfg, 1);
         assert_ne!(first, second);
         // First window is grid-placed (offset by grid_margin); the second no
         // longer fits any grid cell and falls back to cascade, which steps
-        // from the monitor origin by `cascade_offset` per already-placed window.
+        // from the monitor origin by `cascade_offset` per window opened so
+        // far this session (the caller's own counter, passed in as `1` here).
         assert_eq!(second.x, cfg.cascade_offset * 2);
         assert_eq!(second.y, cfg.cascade_offset * 2);
     }
 
     #[test]
+    fn cascade_step_keeps_advancing_even_if_the_previous_window_closed() {
+        // The actual reported bug this counter exists to fix: opening one
+        // window at a time (closing each before the next) used to reset
+        // `existing` to empty every time, so `step` - driven by `existing.
+        // len()` - was always 0 regardless of how many windows had already
+        // been opened-and-closed. A cascade_step the caller keeps
+        // incrementing across the session, independent of what is
+        // currently open, is what actually fixes it. Calls `cascade`
+        // directly (not `place`): `place`'s own grid-first fallback would
+        // succeed for an empty `existing` regardless of this test's own
+        // point (a grid's cell *count* legitimately does depend on live
+        // occupancy - see `place`'s own doc comment on why only `cascade`
+        // takes this counter), so a `place`-level test couldn't actually
+        // isolate cascade's own behavior here.
+        let cfg = PlacementConfig::default();
+        let first = SmartPlacement::cascade(&monitor(), (400, 300), &cfg, 0);
+        let second = SmartPlacement::cascade(&monitor(), (400, 300), &cfg, 1);
+        assert_ne!(first, second, "an unchanged cascade_step of 0 vs 1 must not collapse to the same spot");
+    }
+
+    #[test]
     fn snap_left_edge_yields_left_half() {
         let cfg = PlacementConfig::default();
         let dragged = Rect::new(2, 100, 400, 300); // x=2 is within threshold of left edge