fix(capture): preserve rounded fractional buffer dimensions

compositor/src/handlers/window_capture.rs

@@ -27,9 +27,34 @@ fn capture_scale(output_scale: f64, buffer_scale: i32, view: SurfaceView) -> f64
     if view.dst.w <= 0 || view.dst.h <= 0 {
         return output_scale;
     }
-    let native_x = view.src.size.w * f64::from(buffer_scale) / f64::from(view.dst.w);
-    let native_y = view.src.size.h * f64::from(buffer_scale) / f64::from(view.dst.h);
-    output_scale.max(native_x).max(native_y)
+    let width = view.src.size.w * f64::from(buffer_scale);
+    let height = view.src.size.h * f64::from(buffer_scale);
+    let logical_width = f64::from(view.dst.w);
+    let logical_height = f64::from(view.dst.h);
+    let matches_native = |scale: f64| {
+        (logical_width * scale).round() == width && (logical_height * scale).round() == height
+    };
+    // Fractional clients round their backing dimensions independently. Taking
+    // the larger density can add a pixel on the other axis and filter every
+    // row or column, even though the client already rendered at this scale.
+    if matches_native(output_scale) {
+        return output_scale;
+    }
+    let scale = output_scale
+        .max(width / logical_width)
+        .max(height / logical_height);
+    if matches_native(scale) {
+        return scale;
+    }
+    // Prefer a common density whose rounded dimensions preserve both axes.
+    // Stay strictly below the upper rounding boundary, including FP error.
+    let upper = ((width + 0.5) / logical_width).min((height + 0.5) / logical_height);
+    let adjusted = upper * (1.0 - 8.0 * f64::EPSILON);
+    if adjusted >= output_scale && matches_native(adjusted) {
+        adjusted
+    } else {
+        scale
+    }
 }
 
 #[derive(Debug, Default)]
@@ -621,6 +646,23 @@ pub(crate) mod tests {
         assert_eq!(capture_scale(1.75, 1, cropped), 1.75);
     }
 
+    #[test]
+    fn sharing_preserves_independently_rounded_fractional_buffers() {
+        for (width, height) in [(3819., 2091.), (3819., 2092.)] {
+            let view = SurfaceView {
+                src: Rectangle::from_size((width, height).into()),
+                dst: (2182, 1195).into(),
+                offset: (0, 0).into(),
+            };
+            for output_scale in [1.25, 1.5, 1.75] {
+                let scale = capture_scale(output_scale, 1, view);
+                assert!(scale >= output_scale);
+                assert_eq!((2182. * scale).round(), width);
+                assert_eq!((1195. * scale).round(), height);
+            }
+        }
+    }
+
     /// Exercise both asynchronous publication boundaries using real resources.
     pub(crate) fn check_deferred_snapshot(
         state: &Ferese,

docs/screen-sharing.md

@@ -35,6 +35,9 @@ the captured surface tree when it exceeds the display scale. For example, a 2×
 application buffer stays at 2× in the stream on a 1.75× display, avoiding a
 fractional downsample before the recording app scales it. The video size follows
 window resizes and backing buffer density changes; closing the window ends sharing.
+At fractional scales, clients can round the two backing dimensions separately.
+Capture preserves those pixel dimensions rather than adding a row or column
+by applying the larger axis density to both axes, which would soften text.
 
 The picker follows your shell colors, font, corners, opacity, and blur.
 

scripts/tests/fixtures/window-capture-scale.c

@@ -18,6 +18,8 @@ static struct xdg_wm_base *wm;
 static struct wp_viewporter *viewporter;
 static struct wl_surface *surface;
 static struct wl_buffer *buffer;
+static int logical_width = 640;
+static int logical_height = 480;
 
 static void ping(void *data, struct xdg_wm_base *base, uint32_t serial) {
     (void)data;
@@ -49,7 +51,7 @@ static const struct wl_registry_listener registry_listener = {global, removed};
 static void configure(void *data, struct xdg_surface *xdg, uint32_t serial) {
     (void)data;
     xdg_surface_ack_configure(xdg, serial);
-    xdg_surface_set_window_geometry(xdg, 0, 0, 640, 480);
+    xdg_surface_set_window_geometry(xdg, 0, 0, logical_width, logical_height);
     wl_surface_attach(surface, buffer, 0, 0);
     wl_surface_damage_buffer(surface, 0, 0, INT32_MAX, INT32_MAX);
     wl_surface_commit(surface);
@@ -73,9 +75,15 @@ int main(int argc, char **argv) {
     if (wl_display_roundtrip(display) < 0 || !compositor || !shm || !wm || !viewporter) return 3;
     int integer = !strcmp(argv[1], "integer");
     int fractional = !strcmp(argv[1], "fractional");
+    int rounded = !strcmp(argv[1], "fractional-rounded");
+    int ceiled = !strcmp(argv[1], "fractional-ceiled");
     int cropped = !strcmp(argv[1], "cropped");
-    int width = fractional ? 1120 : 1280;
-    int height = fractional ? 840 : 960;
+    if (rounded || ceiled) {
+        logical_width = 2182;
+        logical_height = 1195;
+    }
+    int width = rounded || ceiled ? 3819 : fractional ? 1120 : 1280;
+    int height = rounded ? 2091 : ceiled ? 2092 : fractional ? 840 : 960;
     int fd = memfd_create("window-capture-scale", MFD_CLOEXEC);
     size_t bytes = (size_t)width * height * 4;
     if (fd < 0 || ftruncate(fd, bytes)) return 4;
@@ -83,7 +91,8 @@ int main(int argc, char **argv) {
     if (pixels == MAP_FAILED) return 5;
     for (int y = 0; y < height; y++)
         for (int x = 0; x < width; x++)
-            pixels[y * width + x] = x % 2 ? 0xffffffff : 0xff000000;
+            pixels[y * width + x] = (x + ((rounded || ceiled) ? y : 0)) % 2
+                ? 0xffffffff : 0xff000000;
     struct wl_shm_pool *pool = wl_shm_create_pool(shm, fd, bytes);
     buffer = wl_shm_pool_create_buffer(pool, 0, width, height, width * 4, WL_SHM_FORMAT_ARGB8888);
     wl_shm_pool_destroy(pool);
@@ -116,7 +125,7 @@ int main(int argc, char **argv) {
         wl_surface_set_buffer_scale(surface, 2);
     } else {
         struct wp_viewport *viewport = wp_viewporter_get_viewport(viewporter, surface);
-        wp_viewport_set_destination(viewport, 640, 480);
+        wp_viewport_set_destination(viewport, logical_width, logical_height);
         if (cropped)
             wp_viewport_set_source(viewport, wl_fixed_from_int(320), wl_fixed_from_int(240),
                                    wl_fixed_from_int(640), wl_fixed_from_int(480));

scripts/tests/test_window_capture_scale_isolated.py

@@ -96,7 +96,9 @@ class WindowCaptureScale(unittest.TestCase):
                                     "an integer 2x host masks the original downsampling bug")
                     for mode, density in [("integer", 2), ("viewport", 2),
                                           ("fractional", 1.75), ("cropped", 1),
-                                          ("subsurface", 2), ("subsurface-outside", 1)]:
+                                          ("subsurface", 2), ("subsurface-outside", 1),
+                                          ("fractional-rounded", 1.75),
+                                          ("fractional-ceiled", 1.75)]:
                         with self.subTest(mode=mode, output_scale=scale):
                             client = launch([str(root / "client"), mode], stdin=subprocess.PIPE)
                             window = wait_for(lambda: next((w for w in call("windows")
@@ -109,6 +111,10 @@ class WindowCaptureScale(unittest.TestCase):
                             ready = json.loads(worker.stdout.readline())
                             expected = (round(640 * max(scale, density)),
                                         round(480 * max(scale, density)))
+                            if mode == "fractional-rounded":
+                                expected = (3819, 2091)
+                            elif mode == "fractional-ceiled":
+                                expected = (3819, 2092)
                             self.assertEqual((ready["width"], ready["height"]), expected,
                                              "window sharing discarded native backing pixels")
                             raw = root / f"{mode}.bgrx"
@@ -134,6 +140,17 @@ class WindowCaptureScale(unittest.TestCase):
                                                  "content outside the window was captured")
                             elif density >= scale:
                                 for index in (0, 2):
+                                    if mode.startswith("fractional-"):
+                                        for y in (0, height // 2, height - 1):
+                                            start = (index * width * height + y * width) * 4
+                                            row = pixels[start:start + width * 4]
+                                            expected_row = b"".join(
+                                                bytes((255, 255, 255, 255)) if (x + y) % 2
+                                                else bytes((0, 0, 0, 255))
+                                                for x in range(width))
+                                            self.assertEqual(row, expected_row,
+                                                             "rounded native pixels were filtered")
+                                        continue
                                     start = (index * width * height + height // 2 * width) * 4
                                     row = pixels[start:start + width * 4]
                                     self.assertEqual(row, bytes((0, 0, 0, 255, 255, 255, 255, 255)) * (width // 2),