Firstly, this is not a new bug. I can replicate it in TextPad 9.9.3, but I believe it's been present for at least a few versions.
I've tried to make a simple test case, but it's possible there are still some other factors at play.
Ensure word wrap is off
Ensure that the window is pretty small. About the default width and height should do. There's some text in the test file and you want to ensure it extends past the edge of the window
Open the provided "Test Case.txt"
Press Ctrl+End to go to the very end of the document
Press Home to go to the start of the line
Press End to go to the end of the line
Press Up twice
Let me know where your cursor is. I would expect it to be on the second-last line of the document, but instead, for me it ends up at the top of the window. Not the top of the document, the top of the window, however tall it currently is.
Using the END key to put the cursor at the end of any line that runs beyond the edge of the window (as you have said, word wrap must be off) and using either the down or up arrow keys to move two lines* will cause the the cursor to jump to position 1,1 (the start of line one) and then the down arrow key will do nothing. You have to either use the right-arrow (at least once) or click somewhere else using the mouse to free the cursor from this position.
I have found the putting the cursor at the end of one of these lines using the mouse (not END key) does not produce this bug. Also I put a caveat on "two lines" because technically it needs to be two non-overflowing lines. If any of the adjacent lines also flow beyond the edge of the window it does not count towards the "two lines"
I first noticed this behavior with v9.8.2. I downloaded 9.9.3 hoping it would be fixed (despite finding no acknowledgement in the release notes) but found the bug still exists so I came to the forum to report it and then found this thread already here.
