Try to disable hardware acceleration in Firefox (you need to close and restart Firefox).

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Thank you. I had already tried that without any effect. I just now tried it again, and restarted FF but the problem is the same.

Try clear your recent history:

• Clear recent history

Also:

• Create a screenshot of the problem
• Firefox in safe mode
• Reset Firefox

I have this problem too, and I just spent some time analyzing it in greater detail.

When you reply to an email in Roundcube, it takes about 5 seconds to load the initial text into the TEXTAREA element. Once it's loaded, causing the TEXTAREA to scroll makes it smear the text into thin vertical lines, like this:

Screenshot 1, streaking phase

After a moment, the vertical lines go away, leaving a blank white textarea with no scroll bar, and a line of displaced text floating outside the textarea at the top:

Screenshot 2, blank phase

This behavior can be triggered with the mousewheel, by dragging the scrollbar, or by using the arrow keys to scroll up or down in the message. The arrow keys skip the vertical lines phase and go straight to the blank phase.

Regardless of how it was triggered, every time it happens it takes about 5 seconds to resolve itself.

I conducted all of my tests in Firefox 27.0.1 on Windows 7 64-bit. I used a clean, blank profile with no extensions or plugins of any kind (I explicitly disabled the default plugins, such as Flash etc). I checked the developer console for error messages, but did not find anything obviously relevant.

One weird thing I noticed was that the problem behavior will stop if you open the developer console with CTRL+SHIFT+K. The textarea will scroll normally until you leave the page and start a fresh email, at which point the problem recurs. Also, if the developer console has been popped out to a window of its own, the problem persists -- opening the console only fixes the problem when it's docked to the window.

This is super weird, and annoying. At a guess, Roundcube's JavaScript may be triggering some weird bug having to do with the painting of window contents. But I couldn't speculate any more thoroughly than that.

EDIT: I should add that disabling hardware acceleration had no effect. Disabling smooth scrolling prevented it form making smeared vertical lines, but it still sat there showing a blank textarea for five seconds every time I scrolled.