User-Agent:       Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:49.0) Gecko/20100101 Firefox/49.0
Build Identifier: 15.12.3

When something tries to display underlined text, the underline doesn't always appear, and when it does, it appears too low, on the top of the next line rather than the bottom of the current one. Sometimes, underlines are visible even after clearing the screen, resulting in what looks like random horizontal lines in the blank space.

Switching windows often affects whether or not underlines are displayed.

Reproducible: Sometimes

Steps to Reproduce:
1. Type 'cat' at the shell prompt in Konsole and press Enter.

2. Press Escape, and then type "[4m" without quotes.

3. Type some lines of text and pay attention to when underlines appear.

4. Press Ctrl+C, and clear the screen.

5. Try opening a man page with underlined text, such as "man chmod". Press 'q' to close the man page. Try this a few times, sometimes scrolling or changing windows while the page is displayed.
Actual Results:  
When typing text in "cat", underlines don't appear until the next line is displayed. Underlines are commonly also seen disappearing upon changing lines. Changing windows causes currently-invisible underlines to be displayed.

When clearing the screen (such as after closing a man page) underlines sometimes remain visible, despite the underlined text no longer being present.

In all cases, underlines appear too low. This is probably the root cause of the issue, as it is outside of the bounding box for the current line.

Expected Results:  
After typing ^[[4m and pressing Enter, text should appear underlined as it is typed. Underlines should always appear under text that is supposed to be underlined, and they should always go away immediately upon clearing the screen. Underlines should appear within the bounds of the line containing the underlined text, even if there is no room to do this without having it touch the text—it is okay if the underline touches the text.

The font I am using is 12-point "PxPlus IBM VGA8" from this site: http://int10h.org/oldschool-pc-fonts/fontlist/