This is similar to RT-16711. It's still reproducible on Linux(tested on OEL6.3 and SLES 11). When trying to delete the last character in composing string, the first backspace closes the candidate window, then the second backspace deletes the last character in composing string.
For TextField, TextArea, HTMLEditor, the behavior is the same. But for native Linux apps, for example the Terminal, it works fine.

A video is attached.