Fix scrolling with partial line corner case (Bug#25792)

Also fix up the scrolling tests so that they don't make so many
assumptions about the current window configuration.

* src/xdisp.c (try_window): Take partial line height into account when
comparing cursor position against scroll margin.

* test/manual/scroll-tests.el (scroll-tests-with-buffer-window): Add
HEIGHT argument, to allow setting up window with exact height and
partial line.
(scroll-tests-display-buffer-with-height): New display-buffer action
function.
(scroll-tests-scroll-margin-over-max):
(scroll-tests--scroll-margin-whole-window): Pass HEIGHT to
`scroll-tests--scroll-margin-whole-window'.
(scroll-tests-conservative-show-trailing-whitespace): New test.
(scroll-tests-scroll-margin-negative): Fix line counting.
(scroll-tests--point-in-middle-of-window-p): Set window height
properly.

1 files changed, 71 insertions, 34 deletions

diff --git a/test/manual/scroll-tests.el b/test/manual/scroll-tests.el
index 1167efd6a66..95647ce0c40 100644
--- a/test/manual/scroll-tests.el
+++ b/test/manual/scroll-tests.el
@@ -53,41 +53,79 @@
     (sit-for 0)
     (should (= 1 (window-start)))))
 
-(defmacro scroll-tests-with-buffer-window (&rest body)
-  (declare (debug t))
+(defun scroll-tests-display-buffer-with-height (buffer alist)
+  (let ((height (alist-get 'window-height alist)))
+    (when height
+      (let* ((window (or (get-buffer-window buffer) (selected-window)))
+             (lines (floor height))
+             (partial (round (* (- height lines) (default-line-height)))))
+        (setq window (cond ((window-in-direction 'above window nil +1))
+                           ((or (window-in-direction 'below window nil -1)
+                                (split-window-below lines))
+                            window)))
+        (set-window-buffer window buffer)
+        (set-window-text-height window lines)
+        (adjust-window-trailing-edge window partial nil t)
+        window))))
+
+(defmacro scroll-tests-with-buffer-window (&optional height &rest body)
+  (declare (debug t) (indent defun))
   `(with-temp-buffer
-     (with-selected-window (display-buffer (current-buffer))
+     (with-selected-window (display-buffer (current-buffer)
+                                           '(scroll-tests-display-buffer-with-height
+                                             . ,(if (numberp height)
+                                                    `((window-height . ,height))
+                                                  (push height body)
+                                                  nil)))
        ,@body)))
 
 (ert-deftest scroll-tests-scroll-margin-0 ()
   (skip-unless (not noninteractive))
   (scroll-tests-with-buffer-window
-   (scroll-tests-up-and-down 0)))
+    (scroll-tests-up-and-down 0)))
 
 (ert-deftest scroll-tests-scroll-margin-negative ()
   "A negative `scroll-margin' should be the same as 0."
   (skip-unless (not noninteractive))
   (scroll-tests-with-buffer-window
-   (scroll-tests-up-and-down -10 0)))
+    (scroll-tests-up-and-down -10 0)))
 
 (ert-deftest scroll-tests-scroll-margin-max ()
   (skip-unless (not noninteractive))
   (scroll-tests-with-buffer-window
-   (let ((max-margin (/ (window-text-height) 4)))
-     (scroll-tests-up-and-down max-margin))))
+    (let ((max-margin (/ (window-text-height) 4)))
+      (scroll-tests-up-and-down max-margin))))
 
 (ert-deftest scroll-tests-scroll-margin-over-max ()
   "A `scroll-margin' more than max should be the same as max."
   (skip-unless (not noninteractive))
-  (scroll-tests-with-buffer-window
-   (set-window-text-height nil 7)
-   (let ((max-margin (/ (window-text-height) 4)))
-     (scroll-tests-up-and-down (+ max-margin 1) max-margin)
-     (scroll-tests-up-and-down (+ max-margin 2) max-margin))))
+  (scroll-tests-with-buffer-window 7
+    (let ((max-margin (/ (window-text-height) 4)))
+      (scroll-tests-up-and-down (+ max-margin 1) max-margin)
+      (scroll-tests-up-and-down (+ max-margin 2) max-margin))))
+
+(ert-deftest scroll-tests-conservative-show-trailing-whitespace ()
+  "Test for Bug#25792."
+  ;; Note: requires partial line to trigger problem.
+  (scroll-tests-with-buffer-window 20.5
+    (let ((show-trailing-whitespace t)
+          (scroll-conservatively 101)
+          (scroll-margin 5))
+      (insert (mapconcat #'number-to-string
+                         (number-sequence 1 200) "\n"))
+      (goto-char 1)
+      (forward-line 15)
+      (sit-for 0)
+      (let ((window-line (count-lines (window-start) (window-point))))
+        (dotimes (_ 10)
+          (call-interactively 'next-line)
+          (sit-for 0)
+          (should (= window-line (count-lines (window-start)
+                                              (window-point)))))))))
 
 (defun scroll-tests--point-in-middle-of-window-p ()
   (= (count-lines (window-start) (window-point))
-     (/ (1- (window-text-height)) 2)))
+     (/ (1- (floor (window-screen-lines))) 2)))
 
 (cl-defun scroll-tests--scroll-margin-whole-window (&key with-line-spacing)
   "Test `maximum-scroll-margin' at 0.5.
@@ -95,27 +133,26 @@ With a high `scroll-margin', this should keep cursor in the
 middle of the window."
   (let  ((maximum-scroll-margin 0.5)
          (scroll-margin 100))
-    (scroll-tests-with-buffer-window
-     (setq-local line-spacing with-line-spacing)
-     ;; Choose an odd number, so there is one line in the middle.
-     (set-window-text-height nil 7)
-     ;; `set-window-text-height' doesn't count `line-spacing'.
-     (when with-line-spacing
-       (window-resize nil (* line-spacing 7) nil nil 'pixels))
-     (erase-buffer)
-     (insert (mapconcat #'number-to-string
-                        (number-sequence 1 200) "\n"))
-     (goto-char 1)
-     (sit-for 0)
-     (call-interactively 'scroll-up-command)
-     (sit-for 0)
-     (should (scroll-tests--point-in-middle-of-window-p))
-     (call-interactively 'scroll-up-command)
-     (sit-for 0)
-     (should (scroll-tests--point-in-middle-of-window-p))
-     (call-interactively 'scroll-down-command)
-     (sit-for 0)
-     (should (scroll-tests--point-in-middle-of-window-p)))))
+    ;; Choose an odd number of lines, so there is a middle line.
+    (scroll-tests-with-buffer-window 7
+      (setq-local line-spacing with-line-spacing)
+      ;; `set-window-text-height' doesn't count `line-spacing'.
+      (when with-line-spacing
+        (window-resize nil (* line-spacing 8) nil nil 'pixels))
+      (erase-buffer)
+      (insert (mapconcat #'number-to-string
+                         (number-sequence 1 200) "\n"))
+      (goto-char 1)
+      (sit-for 0)
+      (call-interactively 'scroll-up-command)
+      (sit-for 0)
+      (should (scroll-tests--point-in-middle-of-window-p))
+      (call-interactively 'scroll-up-command)
+      (sit-for 0)
+      (should (scroll-tests--point-in-middle-of-window-p))
+      (call-interactively 'scroll-down-command)
+      (sit-for 0)
+      (should (scroll-tests--point-in-middle-of-window-p)))))
 
 (ert-deftest scroll-tests-scroll-margin-whole-window ()
   (skip-unless (not noninteractive))