TEdit: Merging two lines of text produces ghost

[hjellinek]

I've reproduced this in master as of May 14, and in rmk88--Split-screen-updates-and-color as of May 28.

Procedure: Open a plain text file with linebreaks in TEdit. (I gave my file the extension ".tedit", but it's not clear that it matters.)

At the end of the first line of text, append a space character.
Select the next line. With the caret at the start of the line, hit backspace to remove the end of line at the end of the previous line and merge the two lines into one.
Note that the first line appears as expected, but the second line is still present.
Selecting characters in the second line moves the caret in the newly-merged first line.

See attached video.

tedit-line-end-delete-error.mov

[pamoroso]

I can reproduce this on Linux Mint 22.1 Cinnamon. VNC Medley, Medley and Maiko both on master.