From 5662d04b2f971bae51ddf4873fad57f457c6b693 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Mon, 18 Aug 2025 11:17:40 +0530
Subject: [PATCH] Fix resized area background color not correct during live
 resize

Have to write sRGB colors rather than linear colors in this case
---
 kitty/glfw.c    | 18 +-----------------
 kitty/shaders.c | 33 ++++++++++++++++++++++++++++-----
 kitty/state.h   |  2 +-
 3 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/kitty/glfw.c b/kitty/glfw.c
index e82b53e1c..c4102056d 100644
--- a/kitty/glfw.c
+++ b/kitty/glfw.c
@@ -299,22 +299,6 @@ cursor_active_callback(GLFWwindow *w, monotonic_t now) {
     }
 }
 
-void
-blank_os_window(OSWindow *osw) {
-    color_type color = OPT(background);
-    if (osw->num_tabs > 0) {
-        Tab *t = osw->tabs + osw->active_tab;
-        if (t->num_windows == 1) {
-            Window *w = t->windows + t->active_window;
-            Screen *s = w->render_data.screen;
-            if (s) {
-                color = colorprofile_to_color(s->color_profile, s->color_profile->overridden.default_bg, s->color_profile->configured.default_bg).rgb;
-            }
-        }
-    }
-    blank_canvas(osw->is_semi_transparent ? osw->background_opacity : 1.0f, color);
-}
-
 static void
 window_pos_callback(GLFWwindow* window, int x UNUSED, int y UNUSED) {
     if (!set_callback_window(window)) return;
@@ -1466,7 +1450,7 @@ create_os_window(PyObject UNUSED *self, PyObject *args, PyObject *kw) {
     bool is_semi_transparent = glfwGetWindowAttrib(glfw_window, GLFW_TRANSPARENT_FRAMEBUFFER);
     // blank the window once so that there is no initial flash of color
     // changing, in case the background color is not black
-    blank_canvas(is_semi_transparent ? OPT(background_opacity) : 1.0f, OPT(background));
+    blank_canvas(is_semi_transparent ? OPT(background_opacity) : 1.0f, OPT(background), true);
     apply_swap_interval(-1);
     // On Wayland the initial swap is allowed only after the first XDG configure event
     if (glfwAreSwapsAllowed(glfw_window)) glfwSwapBuffers(glfw_window);
diff --git a/kitty/shaders.c b/kitty/shaders.c
index 7c16e9d18..cb9b991df 100644
--- a/kitty/shaders.c
+++ b/kitty/shaders.c
@@ -805,7 +805,7 @@ render_a_bar(const UIRenderData *ui, WindowBarData *bar, PyObject *title, bool a
     const unsigned sh = ui->full_framebuffer_height;
     // first blank the area to be drawn to background
     enable_scissor_using_top_left_origin(border_rect, sh);
-    blank_canvas(1.f, bg);
+    blank_canvas(ui->os_window->is_semi_transparent ? ui->os_window->background_opacity : 1.f, bg, false);
     disable_scissor();
     // then draw the rendered text
     save_viewport_using_top_left_origin(
@@ -1000,11 +1000,18 @@ draw_cells_with_layers(const UIRenderData *ui, ssize_t vao_idx) {
 }
 
 void
-blank_canvas(float background_opacity, color_type color) {
-    // See https://github.com/glfw/glfw/issues/1538 for why we use pre-multiplied alpha
-#define C(shift) srgb_color((color >> shift) & 0xFF)
-    glClearColor(C(16), C(8), C(0), background_opacity);
+blank_canvas(float background_opacity, color_type color_in_srgb, bool for_final_output) {
+    if (for_final_output) {
+        // we need to write pre-multiplied sRGB color to framebuffer
+#define C(shift) ((((color_in_srgb >> shift) & 0xFF) / 255.f) * background_opacity)
+        glClearColor(C(16), C(8), C(0), background_opacity);
 #undef C
+    } else {
+        // we need to write pre-multiplied linear color to framebuffer
+#define C(shift) srgb_color((color_in_srgb >> shift) & 0xFF) * background_opacity
+        glClearColor(C(16), C(8), C(0), background_opacity);
+#undef C
+    }
     glClear(GL_COLOR_BUFFER_BIT);
 }
 
@@ -1222,6 +1229,22 @@ draw_resizing_text(OSWindow *w) {
     }
 }
 
+void
+blank_os_window(OSWindow *osw) {
+    color_type color = OPT(background);
+    if (osw->num_tabs > 0) {
+        Tab *t = osw->tabs + osw->active_tab;
+        if (t->num_windows == 1) {
+            Window *w = t->windows + t->active_window;
+            Screen *s = w->render_data.screen;
+            if (s) {
+                color = colorprofile_to_color(s->color_profile, s->color_profile->overridden.default_bg, s->color_profile->configured.default_bg).rgb;
+            }
+        }
+    }
+    blank_canvas(osw->is_semi_transparent ? osw->background_opacity : 1.0f, color, true);
+}
+
 static void
 start_os_window_rendering(OSWindow *os_window) {
     if (os_window->live_resize.in_progress) {
diff --git a/kitty/state.h b/kitty/state.h
index 969aa6451..1e46ac46f 100644
--- a/kitty/state.h
+++ b/kitty/state.h
@@ -407,7 +407,7 @@ void free_texture(uint32_t*);
 void free_framebuffer(uint32_t*);
 void send_image_to_gpu(uint32_t*, const void*, int32_t, int32_t, bool, bool, bool, RepeatStrategy);
 void send_sprite_to_gpu(FONTS_DATA_HANDLE fg, sprite_index, pixel*, sprite_index);
-void blank_canvas(float, color_type);
+void blank_canvas(float, color_type, bool);
 void blank_os_window(OSWindow *);
 void set_os_window_chrome(OSWindow *w);
 FONTS_DATA_HANDLE load_fonts_data(double, double, double);