master 141425c: Make `pixel-scroll-precision-scroll-up' slightly more robust

[Po Lu]

branch: master
commit 141425ce3b8646d589f6a3aaf16d981821b32631
Author: Po Lu <luangruo@yahoo.com>
Commit: Po Lu <luangruo@yahoo.com>

    Make `pixel-scroll-precision-scroll-up' slightly more robust
    
    * lisp/pixel-scroll.el (pixel-scroll-precision-scroll-up):
    Subtract from existing vscroll if feasible.
---
 lisp/pixel-scroll.el | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/lisp/pixel-scroll.el b/lisp/pixel-scroll.el
index 131519a..84e1f66 100644
--- a/lisp/pixel-scroll.el
+++ b/lisp/pixel-scroll.el
@@ -432,21 +432,31 @@ the height of the current window."
       (vertical-motion -1)
       (setq current-y (- current-y (line-pixel-height)))))
   (let ((current-vscroll (window-vscroll nil t)))
-    (setq delta (- delta current-vscroll))
-    (set-window-vscroll nil 0 t))
-  (while (> delta 0)
-    (let ((position (pixel-point-and-height-at-unseen-line)))
-      (set-window-start nil (car position) t)
-      (setq delta (- delta (cdr position)))))
-  (when (< delta 0)
-    (when-let* ((desired-pos (posn-at-x-y 0 (+ (- delta)
-                                         (window-tab-line-height)
-                                         (window-header-line-height))))
-               (desired-start (posn-point desired-pos))
-               (desired-vscroll (cdr (posn-object-x-y desired-pos))))
-      (unless (eq (window-start) desired-start)
-        (set-window-start nil desired-start t))
-      (set-window-vscroll nil desired-vscroll t))))
+    (if (<= delta current-vscroll)
+        (set-window-vscroll nil (- current-vscroll delta) t)
+      (setq delta (- delta current-vscroll))
+      (set-window-vscroll nil 0 t)
+      (while (> delta 0)
+        (let ((position (pixel-point-and-height-at-unseen-line)))
+          (unless (cdr position)
+            (signal 'beginning-of-buffer nil))
+          (set-window-start nil (car position) t)
+          ;; If the line above is taller than the window height (i.e. there's
+          ;; a very tall image), keep point on it.
+          (when (> (cdr position) (window-text-height nil t))
+            (let ((vs (window-vscroll nil t)))
+              (goto-char (car position))
+              (set-window-vscroll nil vs t)))
+          (setq delta (- delta (cdr position)))))
+      (when (< delta 0)
+        (when-let* ((desired-pos (posn-at-x-y 0 (+ (- delta)
+                                                  (window-tab-line-height)
+                                                  
(window-header-line-height))))
+                   (desired-start (posn-point desired-pos))
+                   (desired-vscroll (cdr (posn-object-x-y desired-pos))))
+          (unless (eq (window-start) desired-start)
+            (set-window-start nil desired-start t))
+          (set-window-vscroll nil desired-vscroll t))))))
 
 ;; FIXME: This doesn't work when there's an image above the current
 ;; line that is taller than the window.