fix(terminal): adjust marks when deleting scrollback lines (#36294)

This also fixes inconsistent scrolling behavior on terminal output when
cursor is in the middle of the buffer and the scrollback is full.

src/nvim/terminal.c

@@ -73,6 +73,7 @@
 #include "nvim/macros_defs.h"
 #include "nvim/main.h"
 #include "nvim/map_defs.h"
+#include "nvim/mark.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
 #include "nvim/memory.h"
@@ -150,10 +151,11 @@ struct terminal {
   // window height has increased) and must be deleted from the terminal buffer
   int sb_pending;
   size_t sb_deleted;                // Lines deleted from sb_buffer.
+  size_t sb_deleted_last;           // Value of sb_deleted on last refresh_scrollback()
 
   char *title;     // VTermStringFragment buffer
-  size_t title_len;    // number of rows pushed to sb_buffer
-  size_t title_size;   // sb_buffer size
+  size_t title_len;
+  size_t title_size;
 
   // buf_T instance that acts as a "drawing surface" for libvterm
   // we can't store a direct reference to the buffer because the
@@ -2141,6 +2143,8 @@ static void adjust_scrollback(Terminal *term, buf_T *buf)
       term->sb_current--;
       xfree(term->sb_buffer[term->sb_current]);
     }
+    mark_adjust_buf(buf, 1, (linenr_T)diff, MAXLNUM, -(linenr_T)diff, true,
+                    kMarkAdjustTerm, kExtmarkUndo);
     deleted_lines_buf(buf, 1, (linenr_T)diff);
   }
 
@@ -2156,6 +2160,11 @@ static void adjust_scrollback(Terminal *term, buf_T *buf)
 // Refresh the scrollback of an invalidated terminal.
 static void refresh_scrollback(Terminal *term, buf_T *buf)
 {
+  linenr_T deleted = (linenr_T)(term->sb_deleted - term->sb_deleted_last);
+  deleted = MIN(deleted, buf->b_ml.ml_line_count);
+  mark_adjust_buf(buf, 1, deleted, MAXLNUM, -deleted, true, kMarkAdjustTerm, kExtmarkUndo);
+  term->sb_deleted_last = term->sb_deleted;
+
   int width, height;
   vterm_get_size(term->vt, &height, &width);