macOS: Implement background blurring

Uses a private API that allows us to control the amount of blurring.
While using a private API is obviously not ideal, it is used by both
iTerm.app and Apple's own Terminal.app, so hopefully it should stick
around. Fixes #6135

glfw/cocoa_platform.h

@@ -139,6 +139,7 @@ typedef struct _GLFWwindowNS
     int             width, height;
     int             fbWidth, fbHeight;
     float           xscale, yscale;
+    int             blur_radius;
 
     // The total sum of the distances the cursor has been warped
     // since the last cursor motion event was processed

glfw/cocoa_window.m

@@ -36,6 +36,8 @@
 #include <string.h>
 
 
+GLFWAPI int glfwCocoaSetBackgroundBlur(GLFWwindow *w, int radius);
+
 static const char*
 polymorphic_string_as_utf8(id string) {
     if (string == nil) return "(nil)";
@@ -1835,6 +1837,7 @@ static bool createNativeWindow(_GLFWwindow* window,
 
     _glfwPlatformGetWindowSize(window, &window->ns.width, &window->ns.height);
     _glfwPlatformGetFramebufferSize(window, &window->ns.fbWidth, &window->ns.fbHeight);
+    if (wndconfig->ns.blur_radius > 0) glfwCocoaSetBackgroundBlur((GLFWwindow*)window, wndconfig->ns.blur_radius);
 
     return true;
 }
@@ -2969,6 +2972,18 @@ GLFWAPI void glfwCocoaRequestRenderFrame(GLFWwindow *w, GLFWcocoarenderframefun
     requestRenderFrame((_GLFWwindow*)w, callback);
 }
 
+GLFWAPI int glfwCocoaSetBackgroundBlur(GLFWwindow *w, int radius) {
+    _GLFWwindow* window = (_GLFWwindow*)w;
+    int orig = window->ns.blur_radius;
+    if (radius > -1 && radius != window->ns.blur_radius) {
+        extern OSStatus CGSSetWindowBackgroundBlurRadius(void* connection, NSInteger windowNumber, int radius);
+        extern void* CGSDefaultConnectionForThread(void);
+        CGSSetWindowBackgroundBlurRadius(CGSDefaultConnectionForThread(), [window->ns.object windowNumber], radius);
+        window->ns.blur_radius = radius;
+    }
+    return orig;
+}
+
 GLFWAPI int glfwGetCurrentSystemColorTheme(void) {
     int theme_type = 0;
     NSAppearance *changedAppearance = NSApp.effectiveAppearance;

glfw/glfw.py

@@ -249,6 +249,7 @@ def generate_wrappers(glfw_header: str) -> None:
     GLFWapplicationwillfinishlaunchingfun glfwSetApplicationWillFinishLaunching(GLFWapplicationwillfinishlaunchingfun callback)
     uint32_t glfwGetCocoaKeyEquivalent(uint32_t glfw_key, int glfw_mods, int* cocoa_mods)
     void glfwCocoaRequestRenderFrame(GLFWwindow *w, GLFWcocoarenderframefun callback)
+    int glfwCocoaSetBackgroundBlur(GLFWwindow *w, int blur_radius)
     void* glfwGetX11Display(void)
     int32_t glfwGetX11Window(GLFWwindow* window)
     void glfwSetPrimarySelectionString(GLFWwindow* window, const char* string)

glfw/glfw3.h

@@ -1034,6 +1034,10 @@ typedef enum {
     SRGB_COLORSPACE = 1,
     DISPLAY_P3_COLORSPACE = 2,
 } GlfwCocoaColorSpaces;
+/*! @brief macOS specific
+ *  [window hint](@ref GLFW_COCOA_BLUR_RADIUS_hint).
+ */
+#define GLFW_COCOA_BLUR_RADIUS 0x00023005
 
 /*! @brief X11 specific
  *  [window hint](@ref GLFW_X11_CLASS_NAME_hint).

glfw/internal.h

@@ -313,6 +313,7 @@ struct _GLFWwndconfig
     struct {
         bool      retina;
         int       color_space;
+        int       blur_radius;
         char      frameName[256];
     } ns;
     struct {

glfw/window.c

@@ -334,6 +334,8 @@ void glfwDefaultWindowHints(void)
     _glfw.hints.window.ns.retina = true;
     // use the default colorspace assigned by the system
     _glfw.hints.window.ns.color_space = 0;
+    // no blur
+    _glfw.hints.window.ns.blur_radius = 0;
 }
 
 GLFWAPI void glfwWindowHint(int hint, int value)
@@ -417,6 +419,9 @@ GLFWAPI void glfwWindowHint(int hint, int value)
         case GLFW_COCOA_COLOR_SPACE:
             _glfw.hints.window.ns.color_space = value;
             return;
+        case GLFW_COCOA_BLUR_RADIUS:
+            _glfw.hints.window.ns.blur_radius = value;
+            return;
         case GLFW_COCOA_GRAPHICS_SWITCHING:
             _glfw.hints.context.nsgl.offline = value ? true : false;
             return;

kitty/glfw.c

@@ -902,6 +902,11 @@ create_os_window(PyObject UNUSED *self, PyObject *args, PyObject *kw) {
 
 #ifdef __APPLE__
     glfwWindowHint(GLFW_COCOA_COLOR_SPACE, OPT(macos_colorspace));
+    if (OPT(background_blur) > 0 && OPT(background_opacity) < 1.f) {
+        glfwWindowHint(GLFW_COCOA_BLUR_RADIUS, MIN(OPT(background_blur), 128));
+    } else {
+        glfwWindowHint(GLFW_COCOA_BLUR_RADIUS, 0);
+    }
 #else
     glfwWindowHintString(GLFW_X11_INSTANCE_NAME, wm_class_name);
     glfwWindowHintString(GLFW_X11_CLASS_NAME, wm_class_class);
@@ -1081,6 +1086,17 @@ os_window_update_size_increments(OSWindow *window) {
     }
 }
 
+void
+update_background_blur(OSWindow *os_window) {
+#ifdef __APPLE__
+    int new_blur_radius = 0;
+    if (os_window->background_opacity < 1.f && OPT(background_blur) > -1) new_blur_radius = OPT(background_blur);
+    glfwCocoaSetBackgroundBlur(os_window->handle, new_blur_radius);
+#else
+    (void)os_window;
+#endif
+}
+
 #ifdef __APPLE__
 static bool
 window_in_same_cocoa_workspace(void *w, size_t *source_workspaces, size_t source_workspace_count) {

kitty/options/definition.py

@@ -1371,6 +1371,17 @@
 '''
     )
 
+opt('background_blur', '0', option_type='int', ctype='int',
+    long_text='''
+Set to a positive value to enable background blur (blurring of the visuals
+behind a transparent window) on platforms that support it. Only takes effect
+when :opt:`background_opacity` is less than one. On macOS, this will also
+control the :italic:`blur radius` (amount of blurring). Setting it to too high
+a value will cause severe performance issues and/or rendering artifacts.
+Usually, values up to 64 work well. Note that this might cause performance issues,
+depending on how the platform implements it, so use with care.
+''')
+
 opt('background_image', 'none',
     option_type='config_or_absolute_path', ctype='!background_image',
     long_text='Path to a background image. Must be in PNG format.'

kitty/state.c

@@ -1093,6 +1093,7 @@ PYWRAP0(apply_options_update) {
         get_platform_dependent_config_values(os_window->handle);
         os_window->background_opacity = OPT(background_opacity);
         os_window->is_damaged = true;
+        update_background_blur(os_window);
         for (size_t t = 0; t < os_window->num_tabs; t++) {
             Tab *tab = os_window->tabs + t;
             for (size_t w = 0; w < tab->num_windows; w++) {

kitty/state.h

@@ -89,6 +89,7 @@ typedef struct {
         float val; AdjustmentUnit unit;
     } underline_position, underline_thickness, strikethrough_position, strikethrough_thickness, cell_width, cell_height, baseline;
     bool show_hyperlink_targets;
+    int background_blur;
 } Options;
 
 typedef struct WindowLogoRenderData {
@@ -365,3 +366,4 @@ void update_ime_position(Window* w, Screen *screen);
 bool update_ime_position_for_window(id_type window_id, bool force, int update_focus);
 void set_ignore_os_keyboard_processing(bool enabled);
 void update_menu_bar_title(PyObject *title UNUSED);
+void update_background_blur(OSWindow *);