X11: Set the WINDOWID environment variable

Fixes #232

CHANGELOG.rst

@@ -45,6 +45,8 @@ version 0.6.0 [future]
 
 - Add an option to control the underline style for URL highlighting on hover
 
+- X11: Set the WINDOWID environment variable 
+
 version 0.5.1 [2017-12-01]
 ---------------------------
 

kitty/child.py

@@ -20,11 +20,12 @@ class Child:
     child_fd = pid = None
     forked = False
 
-    def __init__(self, argv, cwd, opts, stdin=None):
+    def __init__(self, argv, cwd, opts, stdin=None, env=None):
         self.argv = argv
         self.cwd = os.path.abspath(os.path.expandvars(os.path.expanduser(cwd or os.getcwd())))
         self.opts = opts
         self.stdin = stdin
+        self.env = env or {}
 
     def fork(self):
         if self.forked:
@@ -37,6 +38,7 @@ class Child:
         if stdin is not None:
             stdin_read_fd, stdin_write_fd = os.pipe()
             remove_cloexec(stdin_read_fd)
+        env = self.env
         pid = os.fork()
         if pid == 0:  # child
             try:
@@ -54,6 +56,7 @@ class Child:
             os.closerange(3, 200)
             # Establish the controlling terminal (see man 7 credentials)
             os.close(os.open(os.ttyname(1), os.O_RDWR))
+            os.environ.update(env)
             os.environ['TERM'] = self.opts.term
             os.environ['COLORTERM'] = 'truecolor'
             if os.path.isdir(terminfo_dir):

kitty/tabs.py

@@ -9,11 +9,13 @@ from functools import partial
 from .borders import Borders
 from .child import Child
 from .config import build_ansi_color_table
-from .constants import WindowGeometry, appname, get_boss, shell_path
+from .constants import (
+    WindowGeometry, appname, get_boss, is_macos, is_wayland, shell_path
+)
 from .fast_data_types import (
     DECAWM, Screen, add_tab, glfw_post_empty_event, remove_tab, remove_window,
     set_active_tab, set_active_window, set_tab_bar_render_data, swap_tabs,
-    swap_windows, viewport_for_window
+    swap_windows, viewport_for_window, x11_window_id
 )
 from .layout import Rect, all_layouts
 from .utils import color_as_int
@@ -113,7 +115,14 @@ class Tab:  # {{{
                 cmd = [shell_path]
             else:
                 cmd = self.args.args or [shell_path]
-        ans = Child(cmd, self.cwd, self.opts, stdin)
+        env = {}
+        if not is_macos and not is_wayland:
+            try:
+                env['WINDOWID'] = str(x11_window_id(self.os_window_id))
+            except Exception:
+                import traceback
+                traceback.print_exc()
+        ans = Child(cmd, self.cwd, self.opts, stdin, env)
         ans.fork()
         return ans