Slow buffer updates with Undotree plugin

[jgvanwyk]

Undotree is a plugin that adds a visual representation of Vim's undo tree. When this tree view is open, certain operations (undo, redo, joining lines) are slow in the GUI, but not in the terminal.

To reproduce:

1. Add the Undotree plugin to ~/.vim/pack/mbbill/opt/undotree so that it will be found by packadd:

mkdir -p ~/.vim/pack/mbbill/opt
git clone https://github.com/mbbill/undotree ~/.vim/pack/mbbill/opt

2. Start MacVim with mvim -u vimrc where vimrc is

set nocompatible
set undofile
packadd undotree

3. Open the undo tree visualiser added by Undotree: :UndotreeToggle
4. Insert 100 lines: 100ii<CR><Esc>
5. Go the first line: gg
6. Join all the lines: Hold J

There should be noticeable lag. Now repeat steps 1-6 except with mvim -v -u vimrc in step 2. Joining the lines should be quite snappy.

I've attached two videos below showing the behaviour in each case, and I've included my system details.

GUI:

GUI.mov

Terminal.app:

Terminal.mov

System details:
OS: macOS Ventura 13.4 arm64
Shell: zsh 5.9
Terminal: Terminal.app
$TERM: xterm-256color
Vim: Downloaded from GitHub Releases

[ychin]

I can reproduce this, but not 100% what the cause is yet. It's probably the renderer. Will probably need to take a look a little later. Currently slot it for the next release (r178).

[ychin]

Seems related to #1259, but not sure.