I was about to report this myself, but using Firestorm. Glad it's not just me :P

Thank you for the report, North and Frionil!

1) Create a default new script.

2) Select the state_entry() line by placing the cursor at the very beginning of line 2, then press shift- down arrow to select to the beginning of line 3.

3) Press shift-tab a few times.

Issue tracked. We have no estimate when it may be implemented. Please see future updates here.

This is tracked at https://github.com/secondlife/viewer/issues/3096

Easy repro: make a cube, put the default new script in it, select the state_entry() line, then try to shift-tab to unindent: the { line following it is also unindented, and the selection grows to touch the llSay line as well. Further shift-tabbing will keep growing the selection and unindenting the wrong lines: the longer the selection, the more the selection keeps growing.

Also affects Firestorm 7.1.11 but doesn't grow the selection there, only garbles indents on the next few lines.