fix(signcol): handle edge case with maximum value

50250542 failed to consider that the maximum passed to buf_signcols
is window scoped whereas the signcols value is buffer scoped. This can
lead to a bug where the signcolumn becomes incorrect if:
- global signcolumn is set to auto:N
- signcolumn in a window is changed locally to auto:M where M > N
- the buffer has a line with M or greater signs.

src/nvim/buffer.c

@@ -5495,11 +5495,19 @@ static int buf_signcols_inner(buf_T *buf, int maximum)
 
 int buf_signcols(buf_T *buf, int maximum)
 {
+  // The maximum can be determined from 'signcolumn' which is window scoped so
+  // need to invalidate signcols if the maximum is greater than the previous
+  // maximum.
+  if (maximum > buf->b_signcols_max) {
+    buf->b_signcols_valid = false;
+  }
+
   if (!buf->b_signcols_valid) {
     int signcols = buf_signcols_inner(buf, maximum);
     // Check if we need to redraw
     if (signcols != buf->b_signcols) {
       buf->b_signcols = signcols;
+      buf->b_signcols_max = maximum;
       redraw_buf_later(buf, NOT_VALID);
     }
 

src/nvim/buffer_defs.h

@@ -864,6 +864,7 @@ struct file_buffer {
   sign_entry_T *b_signlist;     // list of placed signs
   int b_signcols;               // last calculated number of sign columns
   bool b_signcols_valid;        // calculated sign columns is valid
+  int b_signcols_max;           // Maximum value b_signcols is valid for.
 
   Terminal *terminal;           // Terminal instance associated with the buffer
 

src/nvim/option.c

@@ -8165,7 +8165,7 @@ int win_signcol_configured(win_T *wp, int *is_fixed)
   }
 
   int needed_signcols = buf_signcols(wp->w_buffer, maximum);
-  int ret = MAX(minimum, needed_signcols);
+  int ret = MAX(minimum, MIN(maximum, needed_signcols));
   assert(ret <= SIGN_SHOW_MAX);
   return ret;
 }