Notepad++ can be quite a useful application for editing code since it has syntax highlighting, it's quite small, and doesn't use much memory. There have always been a few problems in terms of the UI being buggy, but all that now seems to be sorted in their latest version 4.9 release. They list the following fixes:

• Add smart highlighting feature (double click a word to highlight all the same word in the document).
• Enhance visual effect of Mark all feature.
• Enhance Incremental search dialog docking.
• Enhance Incremental search : add Highlight all feature.
• Add auto-hide menu feature (IE7 style menu - Alt or F10 to toggle).
• Enhance tool bar GUI usability (display a chevron while some tool icons hidden).
• Add style transparency feature, right click on color to enable.
• Fix the files not opening by DnD bug.
• Enhance Find Replace dialog : Add "Extended" option - search (and replace) for tabs(\t), newline(\n\r), and a characters by it's value (\o, \x, \b, \d, \t, \n, \r and \\).
• Bug fixed : Find dialog always scrolls text into view now.
• Add places bar in save as dialog.
• Fix non-recognized relative path bug in command line mode (under Dos prompt).
• Add 2 messages for doc monitor plugin.
• Fix the transparency bug (in Styler Configurator).
• Make search in hidden directory (for Find in files) optional.
• Fix the replace bug while replacing nothing.
• Fix the F3 searching direction bug.

I've been using it all day and so far so good, no problems with the UI.