Subject: bug#46557: 28.0.50; Visual glitches in text mode when tab-bar visibility is altered at load time

From: martin rudalics <[email-masked]>
Date: Mon, 10 May 2021 10:24:36 +0200

>> The following recipe creates a broken visual appearance of emacs in text
 >> mode, at least on GNU/Linux:
 >>
 >> 1) Create a test.el file with these lines:
 >>
 >> (set-frame-parameter nil 'tab-bar-lines 0)
 >> (set-frame-parameter nil 'tab-bar-lines 1)
 >> (set-frame-parameter nil 'tab-bar-lines 0)
 >
 > This bug report can't be closed, because it seems
 > this problem is related to https://debbugs.gnu.org/46827#153
 > where setting tab-bar-lines to 1 changes tab-bar-lines to 2.
 > Reproducible test case:
 >
 > (progn
 >    (set-frame-parameter nil 'tab-bar-lines 1)
 >    (let ((lines (frame-parameter nil 'tab-bar-lines)))
 >      (sit-for 0)
 >      (list lines (frame-parameter nil 'tab-bar-lines))))
 >
 > returns (1 2)

But IIUC the latter holds for a GUI only.  You cannot get two lines on a
TTY unless you wrap the tab bar (if that is possible on a TTY).

martin