Lines are suddenly changing indentation when I edit text or move the cursor through the editor.

This might happen if you don’t use a monospaced font ( e.g: Courier ). To change the editor font:

Go to Window ‣ Preferences.
From here, go to: General ‣ Appearance ‣ Colors and Fonts.
In the window on the right side choose VHDL Language ‣ Editor ‣ VHDL Language Text Font.
Press Edit and check that the font you are using is monospaced. If it isn’t, choose one from the list ( ex: Courier, Monospace).
Ok ‣ Apply