bug#38038: 27.0.50; Emacs freezes opening tabs.

[martin rudalics]

> 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