Vertical scrollbar disabled after undo

[danny]

TextPad 8.15.1 64 bit on Windows 10

Show line numbers is checked

Create a text file with 1 number on each line like this
1
2
3
4
5

Keep entering numbers until the vertical scroll bar is enabled
Enter about 5 more numbers after the scroll bar is enabled
Move cursor to the last line

Do a Replace All with Regular expression checked
Find what: \n
Replace with: ,

This should turn the numbers into a comma separated list and should show in the first line
But after clicking Replace All button, the first line became invisible, even the line number disappeared

Press Ctrl+Z to undo, the numbers are showing again, but the vertical scroll bar is disabled, even though there are more numbers at the bottom
The vertical scroll bar is reenabled after restore or maximize TextPad

[AmigoJack]

Is this unbound to which View > Appearance you use?

Cannot reproduce with 8.4.2x64 on Win7x64 with "Default" Appearance:

[danny]

This may not be in older versions though. I am using 64 bit version 8.15.1

I was using dark theme. I am able to reproduce using default theme as well.