display: damage-rect list + tile-grid trackers, vectorizable pixel loops, wide WC stores

Tearing mitigation for the GOP floor, attacking the copy window from three sides:

- Damage is no longer one bounding box. Two trackers, A/B-switchable at
  compile time (display.zig damage_mode): DamageList (free-form dirty rects,
  overlap-merged) and TileGrid (fixed 64-px tiles, exact O(1) marking, runs
  coalesced back into rects). Far-apart changes — the cursor here, an
  animating layer there — no longer unite into one huge repaint.
- fillRect/composite/blitTile now work in row spans (@memset/@memcpy), so
  the compiler vectorizes them and ReleaseSafe bounds checks drop to per-row.
- The back->front present streams 8-byte volatile stores (presentSpan);
  Backend.present takes the rect list, so each present copies only what
  changed, faster.

Host tests cover both trackers; the display QEMU cases all pass.

system/services/display/backend.zig

@@ -110,22 +110,48 @@ pub const Gop = struct {
         };
     }
 
-    /// Stream the damaged rectangle from the back buffer to the write-combining LFB, row by
+    /// Stream each damaged rectangle from the back buffer to the write-combining LFB, row
-    /// row (sequential writes — what WC memory wants; the LFB is never read).
+    /// by row (sequential writes — what WC memory wants; the LFB is never read). The rows
-    pub fn present(self: *const Gop, damage: Rect) void {
+    /// are copied by `presentSpan` below, which widens the stores by hand: `volatile`
-        const c = damage.intersect(.{ .x = 0, .y = 0, .w = @intCast(self.width), .h = @intCast(self.height) });
+    /// keeps the compiler from eliding or reordering framebuffer writes, but it also
-        if (c.isEmpty()) return;
+    /// forbids it from merging them, so a naive per-pixel loop is stuck at one 4-byte
-        var y: i32 = c.y;
+    /// store per iteration. Keeping each copy small (the damage list) and each store wide
-        while (y < c.bottom()) : (y += 1) {
+    /// shrinks the window in which scanout can sample a half-written frame.
-            const off = @as(usize, @intCast(y)) * self.pitch;
+    pub fn present(self: *const Gop, damage: []const Rect) void {
-            const src: [*]const u32 = @ptrCast(@alignCast(self.back + off));
+        const bounds = Rect{ .x = 0, .y = 0, .w = @intCast(self.width), .h = @intCast(self.height) };
-            const dst: [*]volatile u32 = @ptrCast(@alignCast(self.front + off));
+        for (damage) |rect| {
-            var x: i32 = c.x;
+            const c = rect.intersect(bounds);
-            while (x < c.right()) : (x += 1) dst[@intCast(x)] = src[@intCast(x)];
+            if (c.isEmpty()) continue;
+            const span: usize = @intCast(c.w);
+            var y: i32 = c.y;
+            while (y < c.bottom()) : (y += 1) {
+                const offset = @as(usize, @intCast(y)) * self.pitch + @as(usize, @intCast(c.x)) * 4;
+                const source: [*]const u32 = @ptrCast(@alignCast(self.back + offset));
+                const front_row: [*]volatile u32 = @ptrCast(@alignCast(self.front + offset));
+                presentSpan(front_row, source, span);
+            }
         }
     }
 };
 
+/// Copy `count` pixels into the write-combining front buffer with 8-byte volatile stores
+/// (plus a 4-byte head/tail where the span isn't 8-aligned — pixel spans are always
+/// 4-aligned). The loads come from the cacheable back buffer and are assembled into a
+/// `u64` in registers, so nothing here reads the front buffer.
+fn presentSpan(destination: [*]volatile u32, source: [*]const u32, count: usize) void {
+    var i: usize = 0;
+    if (i < count and (@intFromPtr(destination) & 7) != 0) {
+        destination[0] = source[0];
+        i = 1;
+    }
+    while (i + 2 <= count) : (i += 2) {
+        const pair = @as(u64, source[i]) | (@as(u64, source[i + 1]) << 32);
+        const wide: *volatile u64 = @ptrCast(@alignCast(destination + i));
+        wide.* = pair;
+    }
+    if (i < count) destination[i] = source[i];
+}
+
 /// A display mode the native backend can switch to.
 pub const Mode = scanout_protocol.Mode;
 
@@ -152,8 +178,9 @@ pub const VirtioGpu = struct {
         return .{ .pixels = self.pixels, .stride = self.stride, .width = self.width, .height = self.height };
     }
     /// Ask the driver to present. The composited pixels are already in the shared surface, so
-    /// this is a single request over `.scanout`; the driver transfers + fenced-flushes.
+    /// this is a single request over `.scanout` regardless of how many damage rectangles
-    pub fn present(self: *const VirtioGpu, damage: Rect) void {
+    /// accumulated; the driver transfers + fenced-flushes the whole frame.
+    pub fn present(self: *const VirtioGpu, damage: []const Rect) void {
         _ = damage;
         var request = scanout_protocol.Request{
             .operation = @intFromEnum(scanout_protocol.Operation.present),
@@ -210,7 +237,7 @@ pub const Backend = union(enum) {
             inline else => |*b| b.surface(),
         };
     }
-    pub fn present(self: *const Backend, damage: Rect) void {
+    pub fn present(self: *const Backend, damage: []const Rect) void {
         switch (self.*) {
             inline else => |*b| b.present(damage),
         }

system/services/display/compositor.zig

@@ -57,6 +57,177 @@ pub const Rect = struct {
     }
 };
 
+/// The dirty screen regions accumulated between presents. Kept as a *list* of rectangles,
+/// not one bounding box: when two small things move far apart — the cursor on one side of
+/// the screen, an animating layer on the other — a single bounding box unites them into a
+/// huge region, and presenting it streams megabytes to the framebuffer for a few thousand
+/// changed pixels. The long copy widens the window in which scanout (or QEMU's display
+/// refresh) samples a half-written frame — visible as tearing and cursor trails. Small
+/// separate rectangles keep each copy, and that window, tight.
+///
+/// A new rectangle that overlaps an existing entry is united into it (repainting a modest
+/// superset is harmless — compositing is idempotent); the grown entry is *not* re-merged
+/// against the rest, so entries may overlap, which costs only a duplicate repaint. When
+/// the table is full the newcomer folds into the last entry — degrading toward the old
+/// bounding-box behaviour instead of dropping damage.
+pub const DamageList = struct {
+    pub const capacity = 16;
+
+    rects: [capacity]Rect = [_]Rect{Rect.empty} ** capacity,
+    count: usize = 0,
+
+    pub fn add(self: *DamageList, r: Rect) void {
+        if (r.isEmpty()) return;
+        for (self.rects[0..self.count]) |*existing| {
+            if (!existing.intersect(r).isEmpty()) {
+                existing.* = existing.unite(r);
+                return;
+            }
+        }
+        if (self.count < capacity) {
+            self.rects[self.count] = r;
+            self.count += 1;
+            return;
+        }
+        self.rects[capacity - 1] = self.rects[capacity - 1].unite(r);
+    }
+
+    pub fn isEmpty(self: *const DamageList) bool {
+        return self.count == 0;
+    }
+
+    pub fn slice(self: *const DamageList) []const Rect {
+        return self.rects[0..self.count];
+    }
+
+    pub fn clear(self: *DamageList) void {
+        self.count = 0;
+    }
+};
+
+/// The alternative damage tracker: a **fixed tile grid**, the scheme browser compositors
+/// and tile-based GPUs use. The screen is divided into `tile_size`-pixel tiles up front;
+/// `add` marks the tiles a rectangle touches (a bit per tile — merging is free and exact,
+/// no heuristics), and `collect` walks the grid turning runs of adjacent dirty tiles into
+/// repaint rectangles (horizontal runs, then equal-span rows merged vertically, so
+/// full-screen damage collapses back to a single rectangle).
+///
+/// Trade-off against `DamageList`: tracking is O(1) with a strictly bounded worst case
+/// (never more than the dirty tiles), but repaints are quantized — a 1-pixel change
+/// repaints a whole tile. Which wins depends on the workload; the display service has a
+/// compile-time switch (`damage_mode`) to compare them.
+pub const TileGrid = struct {
+    pub const tile_size = 64;
+    pub const maximum_columns = 128; // supports screens up to 8192 px wide…
+    pub const maximum_rows = 128; // …and 8192 px tall (beyond that, edge tiles stretch)
+    pub const maximum_tiles = maximum_columns * maximum_rows;
+    /// The most rectangles `collect` produces; extras fold into the last (never dropped).
+    pub const maximum_rects = 64;
+
+    width: u32 = 0,
+    height: u32 = 0,
+    columns: u32 = 0,
+    rows: u32 = 0,
+    dirty_count: u32 = 0,
+    dirty: [maximum_tiles]bool = [_]bool{false} ** maximum_tiles,
+
+    /// Size the grid for a screen. Also clears it — callers reset on a geometry change,
+    /// where the mode-set paths damage the whole new screen anyway.
+    pub fn reset(self: *TileGrid, width: u32, height: u32) void {
+        self.width = width;
+        self.height = height;
+        self.columns = @min((width + tile_size - 1) / tile_size, maximum_columns);
+        self.rows = @min((height + tile_size - 1) / tile_size, maximum_rows);
+        self.clear();
+    }
+
+    pub fn matches(self: *const TileGrid, width: u32, height: u32) bool {
+        return self.width == width and self.height == height;
+    }
+
+    pub fn isEmpty(self: *const TileGrid) bool {
+        return self.dirty_count == 0;
+    }
+
+    pub fn clear(self: *TileGrid) void {
+        @memset(&self.dirty, false);
+        self.dirty_count = 0;
+    }
+
+    /// Mark every tile `r` touches. Clips to the screen first, so out-of-range
+    /// rectangles are harmless.
+    pub fn add(self: *TileGrid, r: Rect) void {
+        const screen = Rect{ .x = 0, .y = 0, .w = @intCast(self.width), .h = @intCast(self.height) };
+        const c = r.intersect(screen);
+        if (c.isEmpty()) return;
+        const column_first: u32 = @intCast(@divTrunc(c.x, tile_size));
+        const row_first: u32 = @intCast(@divTrunc(c.y, tile_size));
+        const column_last: u32 = @min(@as(u32, @intCast(@divTrunc(c.right() - 1, tile_size))), self.columns - 1);
+        const row_last: u32 = @min(@as(u32, @intCast(@divTrunc(c.bottom() - 1, tile_size))), self.rows - 1);
+        var row = row_first;
+        while (row <= row_last) : (row += 1) {
+            var column = column_first;
+            while (column <= column_last) : (column += 1) {
+                const index = row * self.columns + column;
+                if (!self.dirty[index]) {
+                    self.dirty[index] = true;
+                    self.dirty_count += 1;
+                }
+            }
+        }
+    }
+
+    /// The screen rectangle covered by tiles [column_first, column_end) of `row`. Edge
+    /// tiles clamp to the true screen size (the last column/row may be partial — or, on a
+    /// screen wider than the grid supports, stretched to cover the remainder).
+    fn tileSpanRect(self: *const TileGrid, column_first: u32, column_end: u32, row: u32) Rect {
+        const x: i32 = @intCast(column_first * tile_size);
+        const y: i32 = @intCast(row * tile_size);
+        const right: i32 = if (column_end >= self.columns) @intCast(self.width) else @intCast(column_end * tile_size);
+        const bottom: i32 = if (row + 1 >= self.rows) @intCast(self.height) else @intCast((row + 1) * tile_size);
+        return .{ .x = x, .y = y, .w = right - x, .h = bottom - y };
+    }
+
+    /// Turn the dirty tiles into repaint rectangles in `out`: coalesce each row's runs of
+    /// adjacent dirty tiles, then merge a run into the rectangle directly above it when
+    /// the spans match — so a dirty block of tiles becomes one rectangle. Returns the
+    /// filled prefix of `out`.
+    pub fn collect(self: *const TileGrid, out: []Rect) []Rect {
+        var count: usize = 0;
+        var row: u32 = 0;
+        while (row < self.rows) : (row += 1) {
+            var column: u32 = 0;
+            while (column < self.columns) {
+                if (!self.dirty[row * self.columns + column]) {
+                    column += 1;
+                    continue;
+                }
+                var run_end = column + 1;
+                while (run_end < self.columns and self.dirty[row * self.columns + run_end]) run_end += 1;
+                const rect = self.tileSpanRect(column, run_end, row);
+                column = run_end;
+
+                var merged = false;
+                for (out[0..count]) |*existing| {
+                    if (existing.x == rect.x and existing.w == rect.w and existing.bottom() == rect.y) {
+                        existing.h += rect.h;
+                        merged = true;
+                        break;
+                    }
+                }
+                if (merged) continue;
+                if (count < out.len) {
+                    out[count] = rect;
+                    count += 1;
+                } else {
+                    out[count - 1] = out[count - 1].unite(rect);
+                }
+            }
+        }
+        return out[0..count];
+    }
+};
+
 /// A block of 32-bit pixels: `pixels` addressed row-major with `stride` pixels between
 /// row starts (≥ width — the framebuffer's stride is pitch/4, a layer's is its width).
 pub const Surface = struct {
@@ -74,15 +245,17 @@ pub const Surface = struct {
     }
 };
 
-/// Fill `rect` of `s` with the native pixel `colour`, clipped to `s`'s bounds.
+/// Fill `rect` of `s` with the native pixel `colour`, clipped to `s`'s bounds. Each row is
+/// one `@memset` over the clipped span, so the compiler vectorizes it and the bounds check
+/// runs once per row, not once per pixel.
 pub fn fillRect(s: Surface, rect: Rect, colour: u32) void {
     const c = rect.intersect(s.bounds());
     if (c.isEmpty()) return;
+    const x0: usize = @intCast(c.x);
+    const span: usize = @intCast(c.w);
     var y: i32 = c.y;
     while (y < c.bottom()) : (y += 1) {
-        const r = s.row(@intCast(y));
+        @memset((s.row(@intCast(y)) + x0)[0..span], colour);
-        var x: i32 = c.x;
-        while (x < c.right()) : (x += 1) r[@intCast(x)] = colour;
     }
 }
 
@@ -94,35 +267,36 @@ pub fn composite(dst: Surface, dx: i32, dy: i32, layer: Surface, clip: Rect) voi
     const on_screen = Rect{ .x = dx, .y = dy, .w = @intCast(layer.width), .h = @intCast(layer.height) };
     const region = on_screen.intersect(clip).intersect(dst.bounds());
     if (region.isEmpty()) return;
+    const span: usize = @intCast(region.w);
+    const dst_x: usize = @intCast(region.x);
+    const src_x: usize = @intCast(region.x - dx);
     var y: i32 = region.y;
     while (y < region.bottom()) : (y += 1) {
-        const src = layer.row(@intCast(y - dy));
+        const source_row = layer.row(@intCast(y - dy)) + src_x;
-        const d = dst.row(@intCast(y));
+        const destination_row = dst.row(@intCast(y)) + dst_x;
-        var x: i32 = region.x;
+        @memcpy(destination_row[0..span], source_row[0..span]);
-        while (x < region.right()) : (x += 1) {
-            d[@intCast(x)] = src[@intCast(x - dx)];
-        }
     }
 }
 
 /// Copy a `w`×`h` tile of native pixels from `src` (raw little-endian bytes, row-major,
-/// tightly packed) into `dst` at (`dx`, `dy`), clipped to `dst`'s bounds. `src` is read
+/// tightly packed) into `dst` at (`dx`, `dy`), clipped to `dst`'s bounds. `src` comes
-/// with `readInt` because it comes straight out of an IPC message buffer and carries no
+/// straight out of an IPC message buffer and carries no alignment guarantee, so each
-/// alignment guarantee. Returns without touching anything if `src` is short.
+/// clipped row is a byte-wise `@memcpy` — which equals the old per-pixel little-endian
+/// `readInt` on every danos target (all little-endian) without the alignment concern.
+/// Returns without touching anything if `src` is short.
 pub fn blitTile(dst: Surface, dx: i32, dy: i32, src: []const u8, w: u32, h: u32) void {
     if (src.len < @as(usize, w) * h * 4) return;
-    var ty: u32 = 0;
+    const region = Rect.init(dx, dy, @intCast(w), @intCast(h)).intersect(dst.bounds());
-    while (ty < h) : (ty += 1) {
+    if (region.isEmpty()) return;
-        const yy = dy + @as(i32, @intCast(ty));
+    const span: usize = @intCast(region.w);
-        if (yy < 0 or yy >= dst.height) continue;
+    const tile_x: usize = @intCast(region.x - dx);
-        const drow = dst.row(@intCast(yy));
+    const dst_x: usize = @intCast(region.x);
-        var tx: u32 = 0;
+    var y: i32 = region.y;
-        while (tx < w) : (tx += 1) {
+    while (y < region.bottom()) : (y += 1) {
-            const xx = dx + @as(i32, @intCast(tx));
+        const tile_y: usize = @intCast(y - dy);
-            if (xx < 0 or xx >= dst.width) continue;
+        const offset = (tile_y * w + tile_x) * 4;
-            const off = (@as(usize, ty) * w + tx) * 4;
+        const destination_row = dst.row(@intCast(y)) + dst_x;
-            drow[@intCast(xx)] = std.mem.readInt(u32, src[off..][0..4], .little);
+        @memcpy(std.mem.sliceAsBytes(destination_row[0..span]), src[offset..][0 .. span * 4]);
-        }
     }
 }
 
@@ -179,6 +353,92 @@ test "composite honours the damage rectangle" {
     try std.testing.expectEqual(@as(u32, 0), back[4 * 8 + 4]); // outside damage
 }
 
+test "damage list keeps disjoint rectangles separate and merges overlap" {
+    var list = DamageList{};
+    list.add(Rect.init(0, 0, 10, 10));
+    list.add(Rect.init(100, 100, 10, 10)); // far away: its own entry
+    try std.testing.expectEqual(@as(usize, 2), list.slice().len);
+    list.add(Rect.init(5, 5, 10, 10)); // overlaps the first: united into it
+    try std.testing.expectEqual(@as(usize, 2), list.slice().len);
+    try std.testing.expectEqual(Rect.init(0, 0, 15, 15), list.slice()[0]);
+    try std.testing.expect(!list.isEmpty());
+    list.clear();
+    try std.testing.expect(list.isEmpty());
+}
+
+test "damage list folds overflow into the last entry instead of dropping it" {
+    var list = DamageList{};
+    var i: i32 = 0;
+    while (i < DamageList.capacity) : (i += 1) {
+        list.add(Rect.init(i * 100, 0, 10, 10)); // disjoint: fills every slot
+    }
+    try std.testing.expectEqual(@as(usize, DamageList.capacity), list.slice().len);
+    const overflow = Rect.init(0, 5000, 10, 10);
+    list.add(overflow);
+    try std.testing.expectEqual(@as(usize, DamageList.capacity), list.slice().len);
+    const last = list.slice()[DamageList.capacity - 1];
+    try std.testing.expect(!last.intersect(overflow).isEmpty()); // still covered
+}
+
+test "damage list ignores empty rectangles" {
+    var list = DamageList{};
+    list.add(Rect.empty);
+    try std.testing.expect(list.isEmpty());
+}
+
+test "tile grid coalesces a run of adjacent tiles into one rectangle" {
+    var grid = TileGrid{};
+    grid.reset(256, 128); // 4×2 tiles of 64 px
+    grid.add(Rect.init(10, 10, 100, 10)); // spans tiles (0,0) and (1,0)
+    var scratch: [TileGrid.maximum_rects]Rect = undefined;
+    const rects = grid.collect(&scratch);
+    try std.testing.expectEqual(@as(usize, 1), rects.len);
+    try std.testing.expectEqual(Rect.init(0, 0, 128, 64), rects[0]);
+}
+
+test "tile grid: full-screen damage collapses back to a single rectangle" {
+    var grid = TileGrid{};
+    grid.reset(1280, 720); // 20×12 tiles; the bottom row is partial (720 = 11*64 + 16)
+    grid.add(Rect.init(0, 0, 1280, 720));
+    var scratch: [TileGrid.maximum_rects]Rect = undefined;
+    const rects = grid.collect(&scratch);
+    try std.testing.expectEqual(@as(usize, 1), rects.len);
+    try std.testing.expectEqual(Rect.init(0, 0, 1280, 720), rects[0]);
+}
+
+test "tile grid keeps far-apart damage as separate rectangles" {
+    var grid = TileGrid{};
+    grid.reset(1280, 720);
+    grid.add(Rect.init(0, 0, 10, 10)); // top-left tile
+    grid.add(Rect.init(1000, 600, 10, 10)); // a far-away tile
+    var scratch: [TileGrid.maximum_rects]Rect = undefined;
+    const rects = grid.collect(&scratch);
+    try std.testing.expectEqual(@as(usize, 2), rects.len);
+}
+
+test "tile grid clamps edge tiles to the true screen size" {
+    var grid = TileGrid{};
+    grid.reset(100, 100); // 2×2 tiles, both partial in each axis
+    grid.add(Rect.init(0, 0, 100, 100));
+    var scratch: [TileGrid.maximum_rects]Rect = undefined;
+    const rects = grid.collect(&scratch);
+    try std.testing.expectEqual(@as(usize, 1), rects.len);
+    try std.testing.expectEqual(Rect.init(0, 0, 100, 100), rects[0]);
+}
+
+test "tile grid clear empties it and reset resizes it" {
+    var grid = TileGrid{};
+    grid.reset(256, 256);
+    grid.add(Rect.init(0, 0, 256, 256));
+    try std.testing.expect(!grid.isEmpty());
+    grid.clear();
+    try std.testing.expect(grid.isEmpty());
+    try std.testing.expect(grid.matches(256, 256));
+    grid.reset(512, 512);
+    try std.testing.expect(!grid.matches(256, 256));
+    try std.testing.expect(grid.isEmpty());
+}
+
 test "blitTile copies a packed tile, clipping and reading unaligned bytes" {
     var back = [_]u32{0} ** (4 * 4);
     const dst = Surface{ .pixels = &back, .stride = 4, .width = 4, .height = 4 };

system/services/display/display.zig

@@ -50,8 +50,10 @@ var pending_modeset_check: bool = false;
 var background: u32 = 0;
 
 /// The layer stack. A fixed table (a compositor has few top-level surfaces during
-/// bring-up); each used slot owns an mmap'd surface. `damage` accumulates the dirty
+/// bring-up); each used slot owns an mmap'd surface. `damage_list` accumulates the dirty
-/// screen region since the last `present`, so a present touches only what changed.
+/// screen rectangles since the last `present`, so a present touches only what changed —
+/// and keeps far-apart changes (the cursor here, an animating layer there) as *separate*
+/// small copies rather than one huge bounding box (see compositor.DamageList).
 const maximum_layers = 16;
 
 const Layer = struct {
@@ -65,7 +67,16 @@ const Layer = struct {
 };
 
 var layers: [maximum_layers]Layer = [_]Layer{.{}} ** maximum_layers;
-var damage: Rect = Rect.empty;
+
+/// Which damage tracker drives `present` — a compile-time A/B switch (both are in
+/// compositor.zig with the trade-off discussion):
+///   .list — free-form dirty rectangles (tight bounds, heuristic merging)
+///   .grid — a fixed 64-px tile grid (exact O(1) merging, tile-quantized repaints)
+const DamageMode = enum { list, grid };
+const damage_mode: DamageMode = .grid;
+
+var damage_list: compositor.DamageList = .{};
+var damage_grid: compositor.TileGrid = .{};
 
 // --- geometry helpers -------------------------------------------------------
 
@@ -78,9 +89,20 @@ fn layerScreenRect(l: *const Layer) Rect {
     return .{ .x = l.x, .y = l.y, .w = @intCast(l.surface.width), .h = @intCast(l.surface.height) };
 }
 
-/// Add `r` (screen coordinates) to the pending damage, clipped to the screen.
+/// Add `r` (screen coordinates) to the pending damage, clipped to the screen. In grid
+/// mode the grid re-sizes itself lazily when the screen geometry changes — every
+/// geometry-changing path (`attach_scanout`, `set_mode`) damages the whole new screen
+/// right after, so damage pending from the old geometry is safely superseded.
 fn addDamage(r: Rect) void {
-    damage = damage.unite(r.intersect(screenRect()));
+    const clipped = r.intersect(screenRect());
+    switch (damage_mode) {
+        .list => damage_list.add(clipped),
+        .grid => {
+            const mode = backend.info();
+            if (!damage_grid.matches(mode.width, mode.height)) damage_grid.reset(mode.width, mode.height);
+            damage_grid.add(clipped);
+        },
+    }
 }
 
 // --- layer operations (called from onMessage and the self-check) ------------
@@ -183,21 +205,29 @@ fn compositeInto(clip: Rect) void {
     }
 }
 
-/// Composite the accumulated damage into the backend's surface, hand it to the backend to
+/// Composite each accumulated damage rectangle into the backend's surface, hand the list
-/// put on screen, then clear the damage. A no-op when nothing is dirty. The frame counter
+/// to the backend to put on screen, then clear the damage. A no-op when nothing is dirty.
-/// advances regardless, so callers can name frames.
+/// The frame counter advances regardless, so callers can name frames.
 fn present() void {
-    const dirty = damage.intersect(screenRect());
+    var scratch: [compositor.TileGrid.maximum_rects]Rect = undefined;
-    if (!dirty.isEmpty()) {
+    const dirty: []const Rect = switch (damage_mode) {
-        compositeInto(dirty);
+        .list => damage_list.slice(),
+        .grid => damage_grid.collect(&scratch),
+    };
+    const had_damage = dirty.len != 0;
+    if (had_damage) {
+        for (dirty) |region| compositeInto(region);
         backend.present(dirty);
     }
-    damage = Rect.empty;
+    switch (damage_mode) {
+        .list => damage_list.clear(),
+        .grid => damage_grid.clear(),
+    }
     frames += 1;
 
     // The first present after a native upgrade confirms the composited frame actually reached
     // the shared scanout surface (the automated stand-in for "it's on screen").
-    if (pending_native_verify and !dirty.isEmpty()) {
+    if (pending_native_verify and had_damage) {
         pending_native_verify = false;
         verifyNativePresent();
     }