; Fix mid-symbol updating/cycling completion preview

This fixes an issue where 'completion-preview-next-candidate'
would fail to take into account the part of the symbol that
follows point (the suffix) when point is at the middle of a
symbol, as well as a similar issue in 'completion-preview--show'
that would manifest with slow 'completion-at-point-functions'.

* lisp/completion-preview.el (completion-preview-next-candidate)
(completion-preview--show): Ensure that the completion preview
remains at the end of a symbol, when updating it while point is
in the middle of that symbol.

* test/lisp/completion-preview-tests.el
(completion-preview-mid-symbol-cycle): New test.  (Bug#68875)

1 files changed, 15 insertions, 0 deletions

diff --git a/test/lisp/completion-preview-tests.el b/test/lisp/completion-preview-tests.el
index 190764e9125..5b2c28bd3dd 100644
--- a/test/lisp/completion-preview-tests.el
+++ b/test/lisp/completion-preview-tests.el
@@ -181,4 +181,19 @@ instead."
       (completion-preview--post-command))
     (completion-preview-tests--check-preview "barbaz" 'exact)))
 
+(ert-deftest completion-preview-mid-symbol-cycle ()
+  "Test cycling the completion preview with point at the middle of a symbol."
+  (with-temp-buffer
+    (setq-local completion-at-point-functions
+                (list
+                 (completion-preview-tests--capf
+                  '("foobar" "foobaz"))))
+    (insert "fooba")
+    (forward-char -2)
+    (let ((this-command 'self-insert-command))
+      (completion-preview--post-command))
+    (completion-preview-tests--check-preview "r")
+    (completion-preview-next-candidate 1)
+    (completion-preview-tests--check-preview "z")))
+
 ;;; completion-preview-tests.el ends here