Some emojis move entire line upwards when rendered at bottom of terminal

[ghost]

I noticed an issue when rendering certain emojis using SetContent. It works fine, unless the Emoji is on the bottom row of the terminal, in which case it moves the whole line upwards.

Tested in Alacritty, Kitty and xfce-terminal and it happens in all of them so I don't think it's terminal specific.

Running the code below, you would expect the bottom line to contain an emoji, then the letter T, but they will be 1 line up instead.

package main

import (
    "github.com/gdamore/tcell/v2"
    //"time"
)

func main() {
    s, err := tcell.NewScreen()
    if err != nil {
        panic(err)
    }
    s.Init()
    defStyle := tcell.StyleDefault.Background(tcell.ColorReset).Foreground(tcell.ColorReset)
    s.SetStyle(defStyle)
    
    termWidth, termHeight := s.Size()
    _, _ = termWidth, termHeight
    
    s.SetContent(4, termHeight-1, 'T', nil, defStyle)
    s.SetContent(1, termHeight-1, '🥹', []rune{' '}, defStyle)
    
    
    for {
        s.Show()
        // Poll event
        ev := s.PollEvent()
        // Process event
        switch ev := ev.(type) {
        case *tcell.EventResize:
            s.Sync()
        case *tcell.EventKey:
            if ev.Key() == tcell.KeyEscape || ev.Key() == tcell.KeyCtrlC {
                s.Fini()
                return
            }
        }
        
    }
}

This is what it looks like:

[gdamore]

I think this is caused by a defect in our hack for the automargin bug. We try to insert the last character in a way that won't cause the automatic margin to scroll. Unfortunately this hack doesn't seem to have properly accounted for the extra space/offset used by full width characters.