Custom shaders: Include border and margin in active_window_padding

active_window_padding only counted padding and cell-size compensation,
so the space a layout reserves for the window border (and margin) was
treated as outside the active window. With dim-inactive-windows this
left a thin dimmed strip around the active window and dimmed the active
border color.

Use render_data.geometry.spaces, which is exactly what the layout
reserved around the window. This respects layouts that do not reserve
border space in minimal border mode, so the active area never extends
into a neighboring window. Add a test that the reserved boxes of
visible windows never overlap and stay within the central area.

docs/changelog.rst

@@ -209,7 +209,7 @@ Detailed list of changes
   color profile
 
 - Custom shaders: The builtin :code:`dim-inactive-windows` shader no longer dims
-  the tab bar and the padding around the active window (:iss:`10524`)
+  the tab bar and the padding, border and margin around the active window (:iss:`10524`)
 
 0.49.1 [2026-09-24]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

docs/custom-shaders.rst

@@ -99,7 +99,7 @@ fraction of the original color retained: ``0.74`` blends in 26% of the target
 color. ``INACTIVE_DIM_COLOR`` is in linear RGB; ``0.2158605`` corresponds to
 sRGB ``#808080``. The default target is black, preserving the existing dimming
 effect. The two optional flags preserve the tab bar and include the active
-window's title bar and padding in its unchanged area. Explicit cell backgrounds
+window's title bar, margin, border and padding in its unchanged area. Explicit cell backgrounds
 and images are blended together with text, while transparency and colors reported to
 terminal applications are preserved. ``animation_step 0`` makes this static
 effect redraw only when content or shader events change.

kitty/shaders.c

@@ -2546,10 +2546,14 @@ run_custom_end_shader(OSWindow *os_window, float sx, float sy, monotonic_t now)
             active_win_geom = w->render_data.geometry;
             if (os_window->viewport_width && os_window->viewport_height) {
                 const float width = (float)os_window->viewport_width, height = (float)os_window->viewport_height;
-                d->active_window_padding[0] = (float)(w->padding.left + w->size_mismatch_padding.left) / width;
-                d->active_window_padding[1] = (float)(w->padding.bottom + w->size_mismatch_padding.bottom) / height;
-                d->active_window_padding[2] = (float)(w->padding.right + w->size_mismatch_padding.right) / width;
-                d->active_window_padding[3] = (float)(w->padding.top + w->size_mismatch_padding.top) / height;
+                // spaces is all the space the layout reserved around the window:
+                // margin, border (only if the layout reserves it), padding and
+                // cell-size compensation.
+                const WindowGeometry *sg = &w->render_data.geometry;
+                d->active_window_padding[0] = (float)sg->spaces.left / width;
+                d->active_window_padding[1] = (float)sg->spaces.bottom / height;
+                d->active_window_padding[2] = (float)sg->spaces.right / width;
+                d->active_window_padding[3] = (float)sg->spaces.top / height;
                 // The content geometry excludes the title bar. Include it before
                 // extending to the outer padding, so the effect boundary cannot
                 // cut through a title bar at either edge of the window.

kitty/shaders/custom/focus-highlight.slang

@@ -22,7 +22,7 @@ static const float INACTIVE_DIM = 0.3;
 static const float3 INACTIVE_DIM_COLOR = float3(0.0);
 // Keep the tab bar outside the effect when requested by a pipeline.
 static const bool DIM_CENTRAL_AREA_ONLY = false;
-// Include the active window's title bar and padding in its unchanged/highlighted area.
+// Include the active window's title bar, margin, border and padding in its unchanged/highlighted area.
 static const bool INCLUDE_WINDOW_PADDING = false;
 
 public float4

kitty/shaders/custom/types.slang

@@ -22,8 +22,8 @@ public struct KittyCustomShaderData {
     // .xy = lower-left corner of the window, .zw = width and height.
     // All components are zero when no active window was rendered this frame.
     public float4 active_window_geometry;
-    // Extents beyond the active content, including a visible title bar, padding
-    // and cell-size compensation.
+    // Extents beyond the active content, including a visible title bar, margin,
+    // border, padding and cell-size compensation.
     // .xy = left/bottom, .zw = right/top, in viewport UV units.
     public float4 active_window_padding;
     // Geometry of the most recently belled window in UV co-ordinates (origin bottom-left).

kitty_tests/layout.py

@@ -1071,6 +1071,38 @@ class TestSplitBorderResize(BaseSplitGeometryTest):
         self.assertIsNone(data.vertical_id)
 
 
+class TestReservedSpaces(BaseSplitGeometryTest):
+    def test_reserved_spaces_do_not_overlap(self):
+        # Custom shaders treat the space a layout reserves around the active
+        # window (WindowGeometry.spaces) as part of it, so the reserved boxes
+        # of visible windows must fit in the central area without overlapping.
+        central = Region((19, 47, 1518, 1146, 1500, 1100))
+        for cls in (Tall, Grid, Horizontal, Stack, Splits):
+            for minimal in (True, False):
+                for num in (1, 2, 3, 5):
+                    layout = create_layout(cls)
+                    windows = create_windows(layout, num=num)
+                    self.stub_dimensions(layout, central, minimal)
+                    layout(windows)
+                    boxes = []
+                    for w in windows:
+                        if not w.is_visible_in_layout:
+                            continue
+                        g = w.geometry
+                        boxes.append((g.left - g.spaces.left, g.top - g.spaces.top, g.right + g.spaces.right, g.bottom + g.spaces.bottom))
+                    with self.subTest(layout=cls.name, minimal=minimal, num=num):
+                        self.assertTrue(boxes)
+                        for left, top, right, bottom in boxes:
+                            self.assertGreaterEqual(left, central.left)
+                            self.assertGreaterEqual(top, central.top)
+                            self.assertLessEqual(right, central.left + central.width)
+                            self.assertLessEqual(bottom, central.top + central.height)
+                        for i, a in enumerate(boxes):
+                            for b in boxes[i + 1 :]:
+                                overlaps = a[0] < b[2] and b[0] < a[2] and a[1] < b[3] and b[1] < a[3]
+                                self.assertFalse(overlaps, f'{a} overlaps {b}')
+
+
 class TestSplitDragGeometry(BaseSplitGeometryTest):
     def make_layout(self, shape, minimal=True, num=4):
         from types import SimpleNamespace