Allow to switch to all empty buffers (read-only as well)

* ace-window.el (aw-window-list): Stop filtering out empty read-only
  buffers.
(aw-empty-buffers-list): New defvar, store empty buffers here.
(aw--done): Return empty buffers to their empty state (they had a single
space inserted in order for overlay to work).
(aw--lead-overlay): Insert a space into an empty buffer.
(aw-select): Move inserting a space into `aw--lead-overlay'.

ace-window.el

@@ -136,15 +136,11 @@ This will make `ace-window' act different from `other-window' for
   (sort
    (cl-remove-if
     (lambda (w)
-      (let ((f (window-frame w))
-            (b (window-buffer w)))
+      (let ((f (window-frame w)))
         (or (not (and (frame-live-p f)
                       (frame-visible-p f)))
             (string= "initial_terminal" (terminal-name f))
-            (aw-ignored-p w)
-            (with-current-buffer b
-              (and buffer-read-only
-                   (= 0 (buffer-size b)))))))
+            (aw-ignored-p w))))
     (cl-case aw-scope
       (global
        (cl-mapcan #'window-list (frame-list)))
@@ -165,6 +161,10 @@ This will make `ace-window' act different from `other-window' for
     (nconc minor-mode-alist
            (list '(ace-window-mode ace-window-mode))))
 
+(defvar aw-empty-buffers-list nil
+  "Store the read-only empty buffers which had to be modified.
+Modify them back eventually.")
+
 (defun aw--done ()
   "Clean up mode line and overlays."
   ;; mode line
@@ -172,13 +172,24 @@ This will make `ace-window' act different from `other-window' for
   ;; background
   (mapc #'delete-overlay aw-overlays-back)
   (setq aw-overlays-back nil)
-  (avy--remove-leading-chars))
+  (avy--remove-leading-chars)
+  (dolist (b aw-empty-buffers-list)
+    (with-current-buffer b
+      (when (string= (buffer-string) " ")
+        (let ((inhibit-read-only t))
+          (delete-region (point-min) (point-max))))))
+  (setq aw-empty-buffers-list nil))
 
 (defun aw--lead-overlay (path leaf)
   "Create an overlay using PATH at LEAF.
 LEAF is (PT . WND)."
+  (let ((wnd (cdr leaf)))
+    (with-selected-window wnd
+      (when (= 0 (buffer-size))
+        (push (current-buffer) aw-empty-buffers-list)
+        (let ((inhibit-read-only t))
+          (insert " ")))
       (let* ((pt (car leaf))
-         (wnd (cdr leaf))
              (ol (make-overlay pt (1+ pt) (window-buffer wnd)))
              (old-str (or
                        (ignore-errors
@@ -206,7 +217,7 @@ LEAF is (PT . WND)."
         (overlay-put ol 'face 'aw-leading-char-face)
         (overlay-put ol 'window wnd)
         (overlay-put ol 'display new-str)
-    (push ol avy--overlays-lead)))
+        (push ol avy--overlays-lead)))))
 
 (defun aw--make-backgrounds (wnd-list)
   "Create a dim background overlay for each window on WND-LIST."
@@ -289,10 +300,6 @@ Amend MODE-LINE to the mode line for the duration of the selection."
                 (t
                  (let ((candidate-list
                         (mapcar (lambda (wnd)
-                                  ;; can't jump if the buffer is empty
-                                  (with-current-buffer (window-buffer wnd)
-                                    (when (= 0 (buffer-size))
-                                      (insert " ")))
                                   (cons (aw-offset wnd) wnd))
                                 wnd-list)))
                    (aw--make-backgrounds wnd-list)