You would have to write a userscript to convert an Alt+Shift+Up/Down “thin” rectangular selection into multiple selections after-the-fact.
Komodo uses the Scintilla editing component. Userscripts can access Scintilla via var scimoz = require('ko/editor').scimoz(). With the returned Scintilla object you would be able to fetch the rectangular selection bounds and then create multiple selections via the Scintilla API (http://scintilla.org/ScintillaDoc.html#MultipleSelectionAndVirtualSpace) Note that Komodo’s Scintilla API methods are slightly different (e.g. SCI_ADDSELECTION -> scimoz.addSelection()). You can use Komodo’s “Console” tab (in the bottom pane) to interactively query the Scintilla API methods to find the correct ones to use.
To enable multiline editing, you have a few methods at your disposal. Firstly, you can use the keyboard shortcuts: On Windows and Linux, press Ctrl+Alt+Arrow Keys for editing multiple lines simultaneously; on macOS, use Cmd+Option+Arrow Keys. Alternatively, you can utilize the mouse by holding down Alt (or Option on macOS) while clicking to insert multiple cursors for editing multiple lines.
Another option is to opt for a text editor supporting multiline editing like Visual Studio Code, Sublime Text, or Notepad++, which usually offer specific keyboard shortcuts for multiline editing. In Visual Studio Code, you can enable this feature by opening the settings editor, searching for “multiCursorModifier,” changing the value to alt or ctrlCmd, and then saving the settings. Once enabled, you can edit multiple lines using Ctrl+Alt+Arrow Keys to edit the current line and the lines above or below it, Alt+Click to insert multi-cursors under your mouse pointer, and Esc to remove all multi-cursors.