Fix tab bar edge strips rendered with opacity < 1 when background_opacity < 1

docs/changelog.rst

@@ -198,6 +198,8 @@ Detailed list of changes
 
 - Make erasing last command robust against commands with no output and commands in the scrollback (:pull:`10201`)
 
+- Fix superfluous margin visible to the left and right of the tab bar when :opt:`background_opacity` is less than one and the window width is not a multiple of the cell width (:iss:`10212`)
+
 
 0.47.4 [2026-06-15]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

kitty/border_vertex.glsl

@@ -52,9 +52,10 @@ void main() {
     float is_window_bg = is_integer_value(rc, WINDOW_BACKGROUND_PLACEHOLDER); // used by window padding areas
     float is_default_bg = is_integer_value(rc, DEFAULT_BG);
     color3 = if_one_then(is_window_bg, window_bg, color3);
-    // Actual border quads must be always drawn opaque
+    // Actual border quads and tab bar edge strips must be always drawn opaque
     float is_not_a_border = zero_or_one(abs(
-        (float(rc) - ACTIVE_BORDER_COLOR) * (float(rc) - INACTIVE_BORDER_COLOR) * (float(rc) - BELL_BORDER_COLOR)
+        (float(rc) - ACTIVE_BORDER_COLOR) * (float(rc) - INACTIVE_BORDER_COLOR) * (float(rc) - BELL_BORDER_COLOR) *
+        (float(rc) - TAB_BAR_EDGE_LEFT_COLOR) * (float(rc) - TAB_BAR_EDGE_RIGHT_COLOR)
     ));
     float final_opacity = if_one_then(is_not_a_border, background_opacity, 1.);
     color_premul = vec4_premul(color3, final_opacity);

kitty/tab_bar.py

@@ -23,7 +23,6 @@ from .fast_data_types import (
     Color,
     Region,
     Screen,
-    background_opacity_of,
     cell_size_for_window,
     get_boss,
     get_options,
@@ -763,11 +762,11 @@ class TabBar:
             if g.bottom < tab_bar.bottom:
                 blank_rects.append(Border(g.left, g.bottom, g.right, tab_bar.bottom, bg))
         else:
-            left_bg = right_bg = bg
-            if opts.tab_bar_margin_color is None and (
-                    opacity := background_opacity_of(self.os_window_id)) is not None and opacity >= 1:
+            if opts.tab_bar_margin_color is None:
                 left_bg = BorderColor.tab_bar_left_edge_color
                 right_bg = BorderColor.tab_bar_right_edge_color
+            else:
+                left_bg = right_bg = bg
             if g.left > tab_bar.left:
                 blank_rects.append(Border(tab_bar.left, g.top, g.left, g.bottom, left_bg))
             if g.right < tab_bar.right: