patch 8.2.5040: scrollbar thumb in scrolled popup not visible

Problem:    Scrollbar thumb in scrolled popup not visible.
Solution:   Show at least one thumb character. (fixes 10492)

src/popupwin.c

@@ -4087,6 +4087,7 @@ update_popups(void (*win_update)(win_T *wp))
 	{
 	    linenr_T	linecount = wp->w_buffer->b_ml.ml_line_count;
 	    int		height = wp->w_height;
+	    int		last;
 
 	    sb_thumb_height = ((linenr_T)height * height + linecount / 2)
 								   / linecount;
@@ -4104,6 +4105,10 @@ update_popups(void (*win_update)(win_T *wp))
 						  / (linecount - wp->w_height);
 	    if (wp->w_topline > 1 && sb_thumb_top == 0 && height > 1)
 		sb_thumb_top = 1;  // show it's scrolled
+	    last = total_height - top_off - wp->w_popup_border[2];
+	    if (sb_thumb_top >= last)
+		// show at least one character
+		sb_thumb_top = last;
 
 	    if (wp->w_scrollbar_highlight != NULL)
 		attr_scroll = syn_name2attr(wp->w_scrollbar_highlight);

src/testdir/dumps/Test_popupwin_scroll_13.dump

@@ -0,0 +1,10 @@
+>1+0&#ffffff0| @73
+|2| @73
+|3| @73
+|4| @20|1+0#0000001#ffd7ff255|9| @27| +0#0000000#a8a8a8255| +0&#ffffff0@21
+|5| @20| +0#4040ff13#ffd7ff255@29| +0#0000000#a8a8a8255| +0&#ffffff0@21
+|6| @20| +0#4040ff13#ffd7ff255@29| +0#0000000#a8a8a8255| +0&#ffffff0@21
+|7| @20| +0#4040ff13#ffd7ff255@29| +0#0000000#0000001| +0&#ffffff0@21
+|8| @73
+|9| @73
+|:|c|a|l@1| |S|c|r|o|l@1|B|o|t@1|o|m|(|)| @36|1|,|1| @10|T|o|p| 

src/testdir/test_popupwin.vim

@@ -2314,8 +2314,8 @@ func Test_popup_scrollbar()
       endif
     endfunc
 
-    def CreatePopup(text: list<string>)
+    def CreatePopup(text: list<string>): number
-      popup_create(text, {
+      return popup_create(text, {
 	    \ minwidth: 30,
 	    \ maxwidth: 30,
 	    \ minheight: 4,
@@ -2342,6 +2342,11 @@ func Test_popup_scrollbar()
       END
       call CreatePopup(text)
     endfunc
+    func ScrollBottom()
+      call popup_clear()
+      let id = CreatePopup(range(20)->map({k, v -> string(v)}))
+      call popup_setoptions(id, #{firstline: 20})
+    endfunc
     map <silent> <F3> :call test_setmouse(5, 36)<CR>
     map <silent> <F4> :call test_setmouse(4, 42)<CR>
     map <silent> <F5> :call test_setmouse(7, 42)<CR>
@@ -2397,6 +2402,10 @@ func Test_popup_scrollbar()
   call term_sendkeys(buf, "j")
   call VerifyScreenDump(buf, 'Test_popupwin_scroll_12', {})
 
+  " check thumb when scrolled all the way down
+  call term_sendkeys(buf, ":call ScrollBottom()\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_scroll_13', {})
+
   " clean up
   call term_sendkeys(buf, "x")
   call StopVimInTerminal(buf)

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5040,
 /**/
     5039,
 /**/