diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9464b78..432f2f3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Added
+- add `window.opacity` for a transparent background (compositor-dependent)
+
 ## [0.10.0] - 2026-07-25
 
 ### Added
diff --git a/README.md b/README.md
index c219819..b998685 100644
--- a/README.md
+++ b/README.md
@@ -159,6 +159,7 @@ width           = 800
 height          = 600
 title           = "mmterm"
 cursor_blink_ms = 500
+opacity         = 1.0             # < 1.0 = transparent background (needs a compositor that supports it)
 
 [shell]
 # program = "/bin/zsh"   # defaults to $SHELL
diff --git a/assets/config.toml b/assets/config.toml
index fbbd7ef..2a757a6 100644
--- a/assets/config.toml
+++ b/assets/config.toml
@@ -11,6 +11,7 @@ title           = "mmterm"
 cursor_blink_ms = 500
 inactive_dim    = 0.55
 detect_urls     = true
+opacity         = 1.0
 
 [shell]
 # program = "/bin/zsh"
diff --git a/doc/SPEC.md b/doc/SPEC.md
index e3f10a6..75fcf58 100644
--- a/doc/SPEC.md
+++ b/doc/SPEC.md
@@ -221,6 +221,7 @@ Screenshot capture is a two-step flow: region selection followed by a name promp
 | window | cursor_blink_ms | uint | `500` |
 | window | inactive_dim | float | `0.55` |
 | window | detect_urls | bool | `true` |
+| window | opacity | float | `1.0` |
 | terminal | scrollback_lines | uint | `10000` (min 100) |
 | shell | program | string? | `$SHELL` |
 | logging | auto_log | bool | `false` |
@@ -228,6 +229,11 @@ Screenshot capture is a two-step flow: region selection followed by a name promp
 | status_bar | right | string | `""` |
 | theme | name | string | `"default"` |
 
+`window.opacity` sets the alpha of the terminal background. Values below `1.0`
+make the window transparent while keeping text and UI chrome fully opaque; this
+requires a compositor that supports transparency and the exact result is
+platform-dependent (X11, Wayland, and macOS handle window alpha differently).
+
 ### Themes
 
 Themes define all terminal and UI colors in a single `.toml` file.
diff --git a/src/config/config_test.rs b/src/config/config_test.rs
index 4bd6877..dbac900 100644
--- a/src/config/config_test.rs
+++ b/src/config/config_test.rs
@@ -134,6 +134,44 @@ fn default_detect_urls_value() {
     assert!(default_detect_urls());
 }
 
+#[test]
+fn default_opacity_value() {
+    assert_eq!(default_opacity(), 1.0);
+    assert_eq!(Config::default().window.opacity, 1.0);
+}
+
+#[test]
+fn opacity_default_applied_when_missing() {
+    let toml = r###"
+[font]
+family = "Mono"
+size = 14.0
+[window]
+width = 800
+height = 600
+title = "t"
+cursor_blink_ms = 500
+[shell]
+[colors]
+background = "#000000"
+foreground = "#ffffff"
+cursor = "#ffffff"
+selection = "#333333"
+palette = []
+"###;
+    let cfg: Config = toml::from_str(toml).expect("parse failed");
+    assert_eq!(cfg.window.opacity, 1.0);
+}
+
+#[test]
+fn opacity_round_trips_through_toml() {
+    let mut cfg = Config::default();
+    cfg.window.opacity = 0.8;
+    let s = toml::to_string_pretty(&cfg).expect("serialize failed");
+    let back: Config = toml::from_str(&s).expect("parse failed");
+    assert_eq!(back.window.opacity, 0.8);
+}
+
 #[test]
 fn detect_urls_default_applied_when_missing() {
     let toml = r###"
diff --git a/src/config/mod.rs b/src/config/mod.rs
index 571c12a..d7c7562 100644
--- a/src/config/mod.rs
+++ b/src/config/mod.rs
@@ -147,6 +147,9 @@ fn default_inactive_dim() -> f32 {
 fn default_detect_urls() -> bool {
     true
 }
+fn default_opacity() -> f32 {
+    1.0
+}
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct WindowConfig {
@@ -158,6 +161,8 @@ pub struct WindowConfig {
     pub inactive_dim: f32,
     #[serde(default = "default_detect_urls")]
     pub detect_urls: bool,
+    #[serde(default = "default_opacity")]
+    pub opacity: f32,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
diff --git a/src/config/tui_config.rs b/src/config/tui_config.rs
index 6060b52..c254313 100644
--- a/src/config/tui_config.rs
+++ b/src/config/tui_config.rs
@@ -33,6 +33,7 @@ const F_AUTO_UPDATE_CHECK: usize = 37;
 const F_AUTO_UPDATE_INSTALL: usize = 38;
 const F_SHELL_INTEGRATION: usize = 39;
 const F_DESKTOP_NOTIFICATIONS: usize = 40;
+const F_OPACITY: usize = 41;
 
 const PALETTE_LABELS: [&str; 16] = [
     "Palette 0  black",
@@ -302,6 +303,14 @@ impl ConfigPanel {
             section: None,
         });
 
+        fields.push(Field {
+            label: "Opacity",
+            hint: "0.0–1.0 window background opacity",
+            value: cfg.window.opacity.to_string(),
+            kind: FieldKind::Float,
+            section: None,
+        });
+
         let mut collapsed = HashSet::new();
         collapsed.insert("Palette");
 
@@ -624,6 +633,10 @@ impl ConfigPanel {
         let detect_urls = get(F_DETECT_URLS)
             .parse::<bool>()
             .map_err(|_| "Invalid detect_urls — use true or false")?;
+        let opacity = get(F_OPACITY)
+            .parse::<f32>()
+            .map_err(|_| "Invalid opacity")?
+            .clamp(0.0, 1.0);
         let shell = {
             let s = get(F_SHELL);
             if s.is_empty() { None } else { Some(s) }
@@ -687,6 +700,7 @@ impl ConfigPanel {
                 cursor_blink_ms: blink_ms,
                 inactive_dim,
                 detect_urls,
+                opacity,
             },
             shell: ShellConfig { program: shell },
             terminal: TerminalConfig { scrollback_lines },
diff --git a/src/config/tui_config_test.rs b/src/config/tui_config_test.rs
index 5f8381b..210590a 100644
--- a/src/config/tui_config_test.rs
+++ b/src/config/tui_config_test.rs
@@ -10,8 +10,8 @@ fn make_panel() -> ConfigPanel {
 #[test]
 fn from_config_has_correct_field_count() {
     let panel = make_panel();
-    // 9 base + 1 scrollback + 2 logging + 1 theme + 4 colors + 16 palette + 1 status_bar + 3 general + 2 updates + 2 shell/notify = 41
-    assert_eq!(panel.fields.len(), 41);
+    // 9 base + 1 scrollback + 2 logging + 1 theme + 4 colors + 16 palette + 1 status_bar + 3 general + 2 updates + 2 shell/notify + 1 opacity = 42
+    assert_eq!(panel.fields.len(), 42);
 }
 
 #[test]
@@ -305,6 +305,7 @@ fn distinct_config() -> Config {
             cursor_blink_ms: 523,
             inactive_dim: 0.42,
             detect_urls: true,
+            opacity: 0.8,
         },
         shell: ShellConfig {
             program: Some("/bin/xyzsh".into()),
@@ -375,6 +376,7 @@ fn field_index_sanity() {
         F_AUTO_UPDATE_INSTALL,
         F_SHELL_INTEGRATION,
         F_DESKTOP_NOTIFICATIONS,
+        F_OPACITY,
     ];
     occupied.extend((0..16).map(|i| F_PALETTE + i));
     occupied.sort_unstable();
@@ -433,6 +435,37 @@ fn build_config_roundtrip_toggles_desktop_notifications() {
     }
 }
 
+#[test]
+fn build_config_preserves_opacity() {
+    let mut panel = make_panel();
+    panel.fields[F_OPACITY].value = "0.75".to_string();
+    if let ConfigAction::Save(cfg) = panel.save() {
+        assert_eq!(cfg.window.opacity, 0.75);
+    } else {
+        panic!("expected Save action");
+    }
+}
+
+#[test]
+fn build_config_clamps_opacity_above_one() {
+    let mut panel = make_panel();
+    panel.fields[F_OPACITY].value = "2.5".to_string();
+    let cfg = panel
+        .build_config()
+        .expect("opacity clamps, does not error");
+    assert_eq!(cfg.window.opacity, 1.0);
+}
+
+#[test]
+fn build_config_clamps_opacity_below_zero() {
+    let mut panel = make_panel();
+    panel.fields[F_OPACITY].value = "-1.0".to_string();
+    let cfg = panel
+        .build_config()
+        .expect("opacity clamps, does not error");
+    assert_eq!(cfg.window.opacity, 0.0);
+}
+
 #[test]
 fn build_config_shell_empty_becomes_none() {
     let mut panel = make_panel();
@@ -680,8 +713,8 @@ fn palette_collapsed_by_default() {
 #[test]
 fn visible_indices_hides_palette_body() {
     let panel = make_panel();
-    // 41 total - 15 palette body fields = 26 visible
-    assert_eq!(panel.visible_indices().len(), 26);
+    // 42 total - 15 palette body fields = 27 visible
+    assert_eq!(panel.visible_indices().len(), 27);
 }
 
 #[test]
@@ -690,7 +723,7 @@ fn toggle_on_palette_header_expands() {
     panel.selected = F_PALETTE;
     panel.toggle_collapse();
     assert!(!panel.collapsed.contains("Palette"));
-    assert_eq!(panel.visible_indices().len(), 41);
+    assert_eq!(panel.visible_indices().len(), 42);
 }
 
 #[test]
@@ -700,7 +733,7 @@ fn toggle_twice_restores_collapsed() {
     panel.toggle_collapse();
     panel.toggle_collapse();
     assert!(panel.collapsed.contains("Palette"));
-    assert_eq!(panel.visible_indices().len(), 26);
+    assert_eq!(panel.visible_indices().len(), 27);
 }
 
 #[test]
@@ -755,10 +788,10 @@ fn move_up_skips_collapsed_palette() {
 #[test]
 fn move_down_at_last_visible_clamps() {
     let mut panel = make_panel();
-    // F_DESKTOP_NOTIFICATIONS is the last field and is always visible
-    panel.selected = F_DESKTOP_NOTIFICATIONS;
+    // F_OPACITY is the last field and is always visible
+    panel.selected = F_OPACITY;
     panel.handle_down();
-    assert_eq!(panel.selected, F_DESKTOP_NOTIFICATIONS);
+    assert_eq!(panel.selected, F_OPACITY);
 }
 
 #[test]
diff --git a/src/renderer/draw_fns.rs b/src/renderer/draw_fns.rs
index c24998b..0709723 100644
--- a/src/renderer/draw_fns.rs
+++ b/src/renderer/draw_fns.rs
@@ -72,6 +72,13 @@ pub(super) fn color_u32(c: Color) -> u32 {
     (0xFF << 24) | ((c.r as u32) << 16) | ((c.g as u32) << 8) | (c.b as u32)
 }
 
+/// Pack a color with an explicit alpha byte in the high 8 bits (`0xAARRGGBB`).
+/// Used for the terminal background when `window.opacity < 1.0`; whether the
+/// alpha is honored depends on the platform compositor (softbuffer).
+pub(super) fn color_u32_with_alpha(c: Color, a: u8) -> u32 {
+    ((a as u32) << 24) | ((c.r as u32) << 16) | ((c.g as u32) << 8) | (c.b as u32)
+}
+
 pub(super) fn dim_color(c: u32, factor: f32) -> u32 {
     let r = (((c >> 16) & 0xFF) as f32 * factor) as u32;
     let g = (((c >> 8) & 0xFF) as f32 * factor) as u32;
@@ -203,6 +210,7 @@ fn resolve_bg_color(
     cursor_color: Color,
     selection_color: Color,
     theme: &ResolvedTheme,
+    bg_alpha: BgAlpha,
 ) -> u32 {
     if is_cursor && cursor_shape == CursorShape::Block {
         color_u32(cursor_color)
@@ -213,7 +221,26 @@ fn resolve_bg_color(
     } else if in_match {
         color_u32(theme.search_match)
     } else {
-        color_u32(bg)
+        // Only cells still on the default background become translucent; a cell
+        // painted by the application keeps its color fully opaque.
+        color_u32_with_alpha(bg, bg_alpha.for_bg(bg))
+    }
+}
+
+/// Window background alpha plus the default background it applies to.
+#[derive(Debug, Clone, Copy)]
+pub(super) struct BgAlpha {
+    pub alpha: u8,
+    pub default_bg: Color,
+}
+
+impl BgAlpha {
+    pub(super) fn for_bg(&self, bg: Color) -> u8 {
+        if bg == self.default_bg {
+            self.alpha
+        } else {
+            0xFF
+        }
     }
 }
 
@@ -228,6 +255,7 @@ pub(super) fn resolve_cell_colors(
     cursor_color: Color,
     selection_color: Color,
     theme: &ResolvedTheme,
+    bg_alpha: BgAlpha,
 ) -> (u32, Color) {
     let (fg, bg) = if cell.reverse {
         (cell.bg, cell.fg)
@@ -253,6 +281,7 @@ pub(super) fn resolve_cell_colors(
         cursor_color,
         selection_color,
         theme,
+        bg_alpha,
     );
     let fg = if (in_match || is_current_match) && !is_cursor {
         SEARCH_MATCH_FG
@@ -429,6 +458,26 @@ mod tests {
         assert_eq!(color_u32(c), 0xFF_12_34_56);
     }
 
+    #[test]
+    fn color_u32_with_alpha_packs_alpha_in_high_byte() {
+        let c = Color::rgb(0x12, 0x34, 0x56);
+        assert_eq!(color_u32_with_alpha(c, 0x80), 0x80_12_34_56);
+        assert_eq!(color_u32_with_alpha(c, 0x00), 0x00_12_34_56);
+        // Full alpha is identical to the opaque packing.
+        assert_eq!(color_u32_with_alpha(c, 0xFF), color_u32(c));
+    }
+
+    #[test]
+    fn bg_alpha_applies_only_to_the_default_background() {
+        let default_bg = Color::rgb(0x1e, 0x1e, 0x2e);
+        let a = BgAlpha {
+            alpha: 0xCC,
+            default_bg,
+        };
+        assert_eq!(a.for_bg(default_bg), 0xCC);
+        assert_eq!(a.for_bg(Color::rgb(0xAA, 0xBB, 0xCC)), 0xFF);
+    }
+
     #[test]
     fn dim_color_reduces_brightness() {
         let c = 0xFF_80_80_80u32;
diff --git a/src/renderer/render_ops.rs b/src/renderer/render_ops.rs
index 8810942..0a3dccd 100644
--- a/src/renderer/render_ops.rs
+++ b/src/renderer/render_ops.rs
@@ -248,6 +248,7 @@ impl App {
                 &self.state.theme,
                 update_badge.as_ref(),
                 self.state.hovered_url.as_deref(),
+                self.state.config.window.opacity,
             );
 
             // Capture screenshot before overlays; views/guards still alive here.
diff --git a/src/renderer/text.rs b/src/renderer/text.rs
index d712105..5d90d77 100644
--- a/src/renderer/text.rs
+++ b/src/renderer/text.rs
@@ -176,12 +176,14 @@ impl Renderer {
         theme: &ResolvedTheme,
         update_badge: Option<&UpdateBadge>,
         hovered_url: Option<&str>,
+        opacity: f32,
     ) {
+        let bg_alpha = (opacity.clamp(0.0, 1.0) * 255.0) as u8;
         let bg_fill = panes
             .first()
             .map(|p| p.grid.default_bg)
             .unwrap_or(theme.background);
-        buf.fill(color_u32(bg_fill));
+        buf.fill(color_u32_with_alpha(bg_fill, bg_alpha));
 
         for pane in panes {
             self.draw_pane(
@@ -192,6 +194,7 @@ impl Renderer {
                 pane.metrics,
                 inactive_dim,
                 theme,
+                bg_alpha,
             );
         }
 
@@ -244,11 +247,21 @@ impl Renderer {
         m: &FontMetrics,
         dim_factor: f32,
         theme: &ResolvedTheme,
+        bg_alpha: u8,
     ) {
         let grid = pane.grid;
+        let bg_alpha = BgAlpha {
+            alpha: bg_alpha,
+            default_bg: grid.default_bg,
+        };
 
         // Pre-fill gutter pixels so they match the pane background.
-        fill_pane_background(buf, buf_width, pane.rect, color_u32(grid.default_bg));
+        fill_pane_background(
+            buf,
+            buf_width,
+            pane.rect,
+            color_u32_with_alpha(grid.default_bg, bg_alpha.alpha),
+        );
 
         let selection_range = if pane.is_active {
             match mode {
@@ -279,6 +292,7 @@ impl Renderer {
                 sb_len,
                 selection_range,
                 row,
+                bg_alpha,
             );
         }
 
@@ -380,6 +394,7 @@ impl Renderer {
         sb_len: usize,
         selection_range: Option<(usize, usize, usize, usize)>,
         row: usize,
+        bg_alpha: BgAlpha,
     ) {
         let [rx, ry, rw, rh] = pane.rect;
         let grid = pane.grid;
@@ -432,6 +447,7 @@ impl Renderer {
                 grid.cursor_color,
                 grid.selection_color,
                 theme,
+                bg_alpha,
             );
 
             self.draw_cell(
diff --git a/src/renderer/text_test.rs b/src/renderer/text_test.rs
index 54a232d..39a3593 100644
--- a/src/renderer/text_test.rs
+++ b/src/renderer/text_test.rs
@@ -191,6 +191,7 @@ fn draw_empty_buffer_does_not_panic() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -237,10 +238,119 @@ fn draw_pane_fills_background_color() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
     assert!(buf.iter().any(|&p| p != 0));
 }
 
+#[test]
+fn draw_with_partial_opacity_does_not_panic() {
+    let mut r = make_renderer();
+    let m = r.make_metrics(Physical(16.0));
+    let (cols, rows) = m.grid_size_for(800, 600u32.saturating_sub(44));
+    let grid = make_grid(cols, rows);
+    let pane = PaneView {
+        grid: &grid,
+        rect: [0, 22, 800, 600 - 44],
+        scroll_offset: 0,
+        is_active: true,
+        show_cursor: false,
+        blink_visible: true,
+        search_matches: &[],
+        search_current: None,
+        hovered_url: None,
+        cursor_shape: CursorShape::Block,
+        metrics: &m,
+    };
+    let mut buf = vec![0u32; 800 * 600];
+    let theme = default_theme();
+    r.draw(
+        &mut buf,
+        800,
+        600,
+        &[pane],
+        &[],
+        &InputMode::Insert,
+        false,
+        &[("shell".to_string(), true, false)],
+        0,
+        0,
+        None,
+        None,
+        0.55,
+        None,
+        false,
+        false,
+        ShellState::Unknown,
+        None,
+        &theme,
+        None,
+        None, // hovered_url
+        0.8,
+    );
+    // Background is filled with a sub-1.0 alpha in the high byte.
+    let bg_alpha = (0.8f32 * 255.0) as u8 as u32;
+    assert!(buf.iter().any(|&p| (p >> 24) == bg_alpha));
+}
+
+#[test]
+fn draw_with_partial_opacity_applies_alpha_inside_the_grid() {
+    // Regression: opacity used to reach only the pane padding, so the alpha was
+    // visible at the window edges while every cell stayed opaque.
+    let mut r = make_renderer();
+    let m = r.make_metrics(Physical(16.0));
+    let (cols, rows) = m.grid_size_for(800, 600u32.saturating_sub(44));
+    let mut grid = make_grid(cols, rows);
+    // Col 0 keeps the default background; col 1 gets an app-set background.
+    grid.write_char(' ');
+    grid.cell_mut(1, 0).bg = Color::rgb(0xAA, 0xBB, 0xCC);
+
+    let pane = make_pane(&grid, &m);
+    let mut buf = vec![0u32; 800 * 600];
+    let theme = default_theme();
+    r.draw(
+        &mut buf,
+        800,
+        600,
+        &[pane],
+        &[],
+        &InputMode::Insert,
+        false,
+        &[("t".to_string(), true, false)],
+        0,
+        0,
+        None,
+        None,
+        0.55,
+        None,
+        false,
+        false,
+        ShellState::Unknown,
+        None,
+        &theme,
+        None,
+        None, // hovered_url
+        0.8,
+    );
+
+    let expected = (0.8f32 * 255.0) as u8 as u32;
+    // Cell (0,0) background pixel: x = PANE_PADDING, y = TAB_BAR_H + PANE_PADDING.
+    let default_px = buf[26 * 800 + 4];
+    assert_eq!(
+        default_px >> 24,
+        expected,
+        "default-background cell must carry the window alpha"
+    );
+    // Cell (1,0) has an explicit background — it stays fully opaque.
+    let colored_px = buf[26 * 800 + 4 + m.cell_width as usize];
+    assert_eq!(
+        colored_px >> 24,
+        0xFF,
+        "app-set cell background must stay opaque"
+    );
+    assert_eq!(colored_px & 0x00FF_FFFF, 0x00AA_BBCC);
+}
+
 #[test]
 fn draw_tab_bar_renders_without_panic() {
     let mut r = make_renderer();
@@ -271,6 +381,7 @@ fn draw_tab_bar_renders_without_panic() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -304,6 +415,7 @@ fn draw_status_bar_renders_without_panic() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -335,6 +447,7 @@ fn draw_status_bar_shell_indicators_render_without_panic() {
         &theme,
         None,
         None, // hovered_url
+        1.0,
     );
     // Something was drawn in the status-bar band (bottom rows).
     assert!(buf.iter().any(|&p| p != 0));
@@ -369,6 +482,7 @@ fn draw_status_bar_pane_title_centered() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 
     let mut buf_without = vec![0u32; 800 * 600];
@@ -394,6 +508,7 @@ fn draw_status_bar_pane_title_centered() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 
     assert!(
@@ -436,6 +551,7 @@ fn draw_status_bar_hovered_url_does_not_panic() {
             &theme,
             None,
             Some(url),
+            1.0,
         );
         assert!(
             buf.iter().any(|&p| p != 0),
@@ -476,6 +592,7 @@ fn draw_status_bar_pane_title_suppressed_in_search() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 
     let mut buf_without = vec![0u32; 800 * 600];
@@ -504,6 +621,7 @@ fn draw_status_bar_pane_title_suppressed_in_search() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 
     assert_eq!(
@@ -566,6 +684,7 @@ fn draw_with_bell_flash_does_not_panic() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -597,6 +716,7 @@ fn draw_with_separator_does_not_panic() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -697,6 +817,7 @@ fn do_draw(r: &mut Renderer, panes: &[PaneView<'_>], mode: &InputMode) {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -868,6 +989,7 @@ fn draw_pane_osc8_link_paints_underline_without_hover() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
     // Underline row is at rect_y + cell_height - 2 (cell at row 0, rect_y = 22)
     let ul_y = (22 + m.cell_height.saturating_sub(2)) as usize;
@@ -963,6 +1085,7 @@ fn draw_pane_reverse_video_swaps_background_to_fg_color() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
     // Cell (0,0) background pixel: x = 4 (PANE_PADDING), y = 22+4 (TAB_BAR_H+PANE_PADDING)
     let px = 4usize;
@@ -1193,6 +1316,7 @@ fn draw_status_bar_search_empty_query_shows_slash() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -1227,6 +1351,7 @@ fn draw_status_bar_search_no_matches_shows_label() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
 }
 
@@ -1406,6 +1531,7 @@ fn pane_padding_leaves_top_left_corner_as_background() {
         &theme,
         None, // update_badge: wired in Task 9
         None, // hovered_url
+        1.0,
     );
     // Every pixel in the top-left padding block must equal bg.
     for dy in 0..PANE_PADDING {
diff --git a/src/winit_handler.rs b/src/winit_handler.rs
index 31dd11f..b219aaa 100644
--- a/src/winit_handler.rs
+++ b/src/winit_handler.rs
@@ -50,6 +50,11 @@ impl ApplicationHandler for App {
         let mut attrs = Window::default_attributes()
             .with_title(self.state.config.window.title.clone())
             .with_window_icon(icon);
+        // Request a transparent window when a sub-1.0 background opacity is set.
+        // Whether it takes effect depends on the platform compositor.
+        if self.state.config.window.opacity < 1.0 {
+            attrs = attrs.with_transparent(true);
+        }
         // A `--maximized` / `--fullscreen` flag takes precedence over the saved
         // session geometry, which in turn overrides the config window size.
         match self.startup_window {