M-x tab-next / tab-previous opens new window and don't reuse existing.

[Anoncheg1]

This command by default create new window and split current frame vertically.
By default with disabled tab-bar-buffer-mode I don't observe this behavior, when I switch tabs it reuse existing window.

[ajrosen]

When tab-bar-buffers-mode is enabled, it uses pop-to-buffer to select buffers. You can customize its behavior by customizing display-buffer-alist. I think you can get the behavior you describe with dispay-buffer-same-window.

(setq display-buffer-alist '((".*" display-buffer-same-window))), or through (customize-variable 'display-buffer-alist)

That would reuse the current window. @Anoncheg1 does that help?

[Anoncheg1]

I switched to tab-line mode for every window where tabs displayed of same major mode as current buffer.

(global-tab-line-mode t)
(setopt tab-line-tabs-function #'tab-line-tabs-mode-buffers)
(global-set-key (kbd "C-M-a") #'tab-line-switch-to-prev-tab) ; shadow beginning-of-defun
(global-set-key (kbd "C-M-e") #'tab-line-switch-to-next-tab) ; shadow end-of-defun