bug#39649: 27.0.60; tab-line doesn't scroll

[Juri Linkov]

>> Seems to fix the issue in a specific situation, viz. after I select a
>> tab manually with the mouse. However, when selecting a buffer that is
>> already visible in the tab-line the tab-line still might get scrolled
>> or if I just scroll the tab-line and change to a non-visible buffer
>> the tab-line doesn't get scrolled. I'm a bit interested in knowing the
>> reasoning to not always scroll the tab-line.
>
> While you're manually scrolling the tab-line using the mouse wheel
> or clicking on arrow buttons, auto-scrolling should be disabled
> because it should not bring the current tab back into view immediately
> on every step of manual scrolling.  Just imagine that you clicked
> on the arrow button to scroll, but it has no effect because auto-scrolling
> immediately moved the tab-line back to its original state.
> This is why auto-scrolling should be disabled during manual scrolling.
>
> Now the question: how to detect the moment when you stop manually
> scrolling?  How would you indicate that manual scrolling is finished,
> and it's time to enable auto-scrolling to bring the currently selected tab
> back into view?  I have no idea.

Actually, I have an idea - the right moment is when the current buffer
changes on the current tab.  This patch is the right way to handle this case:

diff --git a/lisp/tab-line.el b/lisp/tab-line.el
index 86ac234f3d..ad2b167e33 100644
--- a/lisp/tab-line.el
+++ b/lisp/tab-line.el
@@ -464,6 +464,13 @@ tab-line-format
                           (window-buffer)
                           (window-parameter nil 'tab-line-hscroll)))
          (cache (window-parameter nil 'tab-line-cache)))
+    ;; Enable auto-hscroll again after it was disabled on manual scrolling.
+    ;; The moment to enable it is when the window-buffer was updated.
+    (when (and tab-line-auto-hscroll       ; if auto-hscroll was enabled
+               (natnump (nth 2 cache-key)) ; non-negative on manual scroll
+               cache                       ; window-buffer was updated
+               (not (equal (nth 1 (car cache)) (nth 1 cache-key))))
+      (set-window-parameter nil 'tab-line-hscroll nil))
     (or (and cache (equal (car cache) cache-key) (cdr cache))
         (cdr (set-window-parameter
               nil 'tab-line-cache