From bc9ae494244e9d08e5352b9cfad09b519398337e Mon Sep 17 00:00:00 2001
From: Daniel Mendler <mail@daniel-mendler.de>
Date: Fri, 1 Nov 2024 11:33:35 +0100
Subject: Use fringe for the scroll bar

This ensures that the scroll bar is always visible, even if wide characters lead
to truncation of the popup content. Before this change, the scroll bar could be
pushed out of the window by the content.
---
 corfu.el                      | 27 ++++++++++++++++++---------
 extensions/corfu-popupinfo.el |  1 -
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/corfu.el b/corfu.el
index ea01161..374b463 100644
--- a/corfu.el
+++ b/corfu.el
@@ -326,8 +326,6 @@ See also the settings `corfu-auto-delay', `corfu-auto-prefix' and
     (outer-border-width . 0)
     (internal-border-width . 1)
     (child-frame-border-width . 1)
-    (left-fringe . 0)
-    (right-fringe . 0)
     (vertical-scroll-bars . nil)
     (horizontal-scroll-bars . nil)
     (menu-bar-lines . 0)
@@ -352,12 +350,12 @@ See also the settings `corfu-auto-delay', `corfu-auto-prefix' and
     (cursor-type . nil)
     (show-trailing-whitespace . nil)
     (display-line-numbers . nil)
-    (left-fringe-width . nil)
-    (right-fringe-width . nil)
+    (left-fringe-width . 0)
+    (right-fringe-width . 0)
     (left-margin-width . 0)
     (right-margin-width . 0)
     (fringes-outside-margins . 0)
-    (fringe-indicator-alist . nil)
+    (fringe-indicator-alist (continuation) (truncation))
     (indicate-empty-lines . nil)
     (indicate-buffer-boundaries . nil)
     (buffer-read-only . t))
@@ -462,6 +460,8 @@ FRAME is the existing frame."
                    `((parent-frame . ,parent)
                      (minibuffer . ,(minibuffer-window parent))
                      (width . 0) (height . 0) (visibility . nil)
+                     (right-fringe . ,right-fringe-width)
+                     (left-fringe . ,left-fringe-width)
                      ,@corfu--frame-parameters))))
     ;; XXX HACK Setting the same frame-parameter/face-background is not a nop.
     ;; Check before applying the setting. Without the check, the frame flickers
@@ -483,6 +483,8 @@ FRAME is the existing frame."
            (should `((background-color
                       . ,(face-attribute 'corfu-default :background nil 'default))
                      (font . ,(frame-parameter parent 'font))
+                     (right-fringe . ,right-fringe-width)
+                     (left-fringe . ,left-fringe-width)
                      ,@corfu--frame-parameters))
            (diff (cl-loop for p in should for (k . v) = p
                           unless (equal (alist-get k is) v) collect p)))
@@ -1021,15 +1023,18 @@ A scroll bar is displayed from LO to LO+BAR."
     (with-current-buffer (corfu--make-buffer " *corfu*")
       (let* ((ch (default-line-height))
              (cw (default-font-width))
+             (bw (ceiling (* cw corfu-bar-width)))
+             (fringe (display-graphic-p))
              (ml (ceiling (* cw corfu-left-margin-width)))
              (mr (ceiling (* cw corfu-right-margin-width)))
-             (bw (ceiling (min mr (* cw corfu-bar-width))))
              (marginl (and (> ml 0) (propertize " " 'display `(space :width (,ml)))))
-             (sbar (when (> bw 0)
+             (sbar (if fringe
+                       #(" " 0 1 (display (right-fringe corfu--bar corfu-bar)))
                      (concat (propertize " " 'display `(space :align-to (- right (,bw))))
                              (propertize " " 'face 'corfu-bar 'display `(space :width (,bw))))))
+             (cbar (and fringe #(" " 0 1 (display (right-fringe corfu--bar corfu-current)))))
              (pos (posn-x-y pos))
-             (width (+ (* width cw) ml mr))
+             (width (+ (* width cw) ml mr (if fringe (- bw) 0)))
              ;; XXX HACK: Minimum popup height must be at least 1 line of the
              ;; parent frame (gh:minad/corfu#261).
              (height (max lh (* (length lines) ch)))
@@ -1042,13 +1047,17 @@ A scroll bar is displayed from LO to LO+BAR."
                     (- yb height lh border border)
                   yb))
              (row 0))
+        (setq right-fringe-width (if fringe bw 0))
+        (when (and (> right-fringe-width 0) (not (fringe-bitmap-p 'corfu--bar)))
+          (define-fringe-bitmap 'corfu--bar []))
         (with-silent-modifications
           (erase-buffer)
           (apply #'insert
            (cl-loop for line in lines collect
                     (let ((str (concat
                                 marginl line
-                                (and lo (<= lo row (+ lo bar)) sbar)
+                                (or (and lo (<= lo row (+ lo bar)) sbar)
+                                    (and (= row curr) cbar))
                                 "\n")))
                       (when (= row curr)
                         (add-face-text-property
diff --git a/extensions/corfu-popupinfo.el b/extensions/corfu-popupinfo.el
index 1115afb..585052b 100644
--- a/extensions/corfu-popupinfo.el
+++ b/extensions/corfu-popupinfo.el
@@ -130,7 +130,6 @@ documentation from the backend is usually expensive."
     (left-margin-width . 1)
     (right-margin-width . 1)
     (word-wrap . t)
-    (fringe-indicator-alist (continuation))
     (char-property-alias-alist (face font-lock-face)))
   "Buffer parameters.")
 
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