mu4e-headers: switch buffer to search results

..except for 'background searches', such as the one after re-indexing.

Fixes #2545.

mu4e/mu4e-headers.el

@@ -783,6 +783,11 @@ present, don't do anything."
 (defconst mu4e~no-matches     "No matching messages found")
 (defconst mu4e~end-of-results "End of search results")
 
+(defvar mu4e--search-background nil
+  "Is this a background search?
+ If so, do not attempt to switch buffers. This variable is to be let-bound
+to t before \"automatic\" searches.")
+
 (defun mu4e--search-execute (expr ignore-history)
   "Search for query EXPR.
 
@@ -810,7 +815,7 @@ true, do *not* update the query history stack."
 
     ;; when the buffer is already visible, select it; otherwise,
     ;; switch to it.
-    (unless (get-buffer-window buf 0)
+    (unless (get-buffer-window buf (if mu4e--search-background 0 nil))
       (mu4e-display-buffer buf t))
     (run-hook-with-args 'mu4e-search-hook expr)
     (mu4e~headers-clear mu4e~search-message)
@@ -1090,7 +1095,8 @@ no user-interaction ongoing."
     ;; otherwise we'd trigger a headers view from out of nowhere.
     (when (and (buffer-live-p (mu4e-get-headers-buffer))
                (window-live-p (get-buffer-window (mu4e-get-headers-buffer) t)))
-      (mu4e-search-rerun))))
+      (let ((mu4e--search-background t))
+        (mu4e-search-rerun)))))
 
 (defcustom mu4e-headers-eldoc-format "“%s” from %f on %d"
   "Format for the `eldoc' string for the current message in the headers buffer.