core: detect what desktop environment the user is using (#4343)

src/cli/version.zig

@@ -3,6 +3,7 @@ const build_options = @import("build_options");
 const Allocator = std.mem.Allocator;
 const builtin = @import("builtin");
 const build_config = @import("../build_config.zig");
+const internal_os = @import("../os/main.zig");
 const xev = @import("xev");
 const renderer = @import("../renderer.zig");
 const gtk = if (build_config.app_runtime == .gtk) @import("../apprt/gtk/c.zig").c else void;
@@ -37,6 +38,7 @@ pub fn run(alloc: Allocator) !u8 {
     try stdout.print("  - renderer   : {}\n", .{renderer.Renderer});
     try stdout.print("  - libxev     : {}\n", .{xev.backend});
     if (comptime build_config.app_runtime == .gtk) {
+        try stdout.print("  - desktop env: {s}\n", .{@tagName(internal_os.desktopEnvironment())});
         try stdout.print("  - GTK version:\n", .{});
         try stdout.print("    build      : {d}.{d}.{d}\n", .{
             gtk.GTK_MAJOR_VERSION,

src/os/desktop.zig

@@ -59,3 +59,29 @@ pub fn launchedFromDesktop() bool {
         else => @compileError("unsupported platform"),
     };
 }
+
+pub const DesktopEnvironment = enum {
+    gnome,
+    macos,
+    other,
+    windows,
+};
+
+/// Detect what desktop environment we are running under. This is mainly used on
+/// Linux to enable or disable GTK client-side decorations but there may be more
+/// uses in the future.
+pub fn desktopEnvironment() DesktopEnvironment {
+    return switch (comptime builtin.os.tag) {
+        .macos => .macos,
+        .windows => .windows,
+        .linux => de: {
+            if (@inComptime()) @compileError("Checking for the desktop environment on Linux must be done at runtime.");
+            // use $XDG_SESSION_DESKTOP to determine what DE we are using on Linux
+            // https://www.freedesktop.org/software/systemd/man/latest/pam_systemd.html#desktop=
+            const de = posix.getenv("XDG_SESSION_DESKTOP") orelse break :de .other;
+            if (std.ascii.eqlIgnoreCase("gnome", de)) break :de .gnome;
+            break :de .other;
+        },
+        else => .other,
+    };
+}

src/os/main.zig

@@ -32,6 +32,7 @@ pub const getenv = env.getenv;
 pub const setenv = env.setenv;
 pub const unsetenv = env.unsetenv;
 pub const launchedFromDesktop = desktop.launchedFromDesktop;
+pub const desktopEnvironment = desktop.desktopEnvironment;
 pub const rlimit = file.rlimit;
 pub const fixMaxFiles = file.fixMaxFiles;
 pub const restoreMaxFiles = file.restoreMaxFiles;