start: Make start-browser synchronous.

There is no point in having it asynchronous, since anything run in parallel
before browser is ready is likely to trigger a race conditions or undefined
behaviour.

It simplifies testing a lot.

source/browser.lisp

@@ -150,6 +150,11 @@ issued by Control+<button1> in a new window.")
     (time:now)
     :export nil
     :documentation "`time:timestamp' of when Nyxt was started.")
+   (startup-promise
+    (lpara:promise)
+    :export nil
+    :accessor nil
+    :documentation "")
    (init-time
     0.0
     :type alex:non-negative-real
@@ -390,6 +395,7 @@ restored."
         (error (c)
           ;; TODO: Clear buffers or back up history?
           (log:warn c)))
+      (lpara:fulfill (slot-value browser 'startup-promise))
       (hooks:run-hook (after-startup-hook browser) browser)
       (funcall* (startup-error-reporter-function browser)))))
 

source/start.lisp

@@ -584,7 +584,8 @@ Finally, run the browser, load URL-STRINGS if any, then run
             ;; is the case with the SLY mrepl thread.
             (bt:make-thread (lambda ()
                               (in-package :nyxt-user)))
-            (ffi-initialize *browser* urls startup-timestamp))))
+            (ffi-initialize *browser* urls startup-timestamp)
+            (lpara:force (slot-value *browser* 'startup-promise)))))
     (quit ()
       :report "Run `nyxt:quit' and try again."
       (quit)