bug#38038: 27.0.50; Emacs freezes opening tabs.

[martin rudalics <rudalics@gmx.at>]

 > And the error says that the window has no buffer, is that right?

I think so but didn't look.  Is it a good idea to signal that error
here in the first place?

 >> where frame #3 indicates that we probably should avoid asking for a buffer
 >> local value in a pseudo window.  Right?
 >
 > Yes.  Does adding that fix the problem, including the original one
 > with the tab bar?

The below makes the bug go away for the tool bar, I didn't try for the
tab bar.

diff --git a/src/w32term.c b/src/w32term.c
index 9da0845836..68ae9be8e7 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -2512,20 +2512,23 @@ w32_draw_glyph_string (struct glyph_string *s)
  		  unsigned long minimum_offset;
  		  BOOL underline_at_descent_line;
  		  BOOL use_underline_position_properties;
-		  Lisp_Object val
-		    = buffer_local_value (Qunderline_minimum_offset,
-		                          s->w->contents);
+		  Lisp_Object val;
+
+		  if (BUFFERP (s->w->contents))
+		    val = buffer_local_value (Qunderline_minimum_offset,
+					      s->w->contents);
  		  if (FIXNUMP (val))
  		    minimum_offset = max (0, XFIXNUM (val));
  		  else
  		    minimum_offset = 1;
-		  val = buffer_local_value (Qx_underline_at_descent_line,
-					    s->w->contents);
+		  if (BUFFERP (s->w->contents))
+		    val = buffer_local_value (Qx_underline_at_descent_line,
+					      s->w->contents);
  		  underline_at_descent_line
  		    = !(NILP (val) || EQ (val, Qunbound));
-		  val
-		    = buffer_local_value (Qx_use_underline_position_properties,
-					  s->w->contents);
+		  if (BUFFERP (s->w->contents))
+		    val = buffer_local_value (Qx_use_underline_position_properties,
+					      s->w->contents);
  		  use_underline_position_properties
  		    = !(NILP (val) || EQ (val, Qunbound));

The bug was apparently introduced by

commit bbf53d99b57bd3d7fc0427d378df2efc6fa10e00
Author: Aaron Jensen <aaronjensen@gmail.com>
Date:   Sat Mar 10 13:14:28 2018 +0200

     Allow underline position variables be buffer-local

     * src/nsterm.m (ns_draw_text_decoration):
     * src/w32term.c (x_draw_glyph_string):
     * src/xterm.c (x_draw_glyph_string): Allow underline-minimum-offset,
     underline-at-descent-line, and x-use-underline-position-properties
     be buffer local variables.  (Bug#30553)
     * src/xdisp.c (syms_of_xdisp) <underline-minimum-offset>: Add DEFSYM.

and probably manifests itself, as noted earlier, in all pseudo windows
that try to draw an underlined glyph string.

I can continue working on this after we decided whether to ask for
pseudo-windowness or just use the BUFFERP predicate as I did.

martin