Win32 linebreak not working inside a textbox?

Explaination:

'\n' or '0x0A' (10 in decimal) -> This character is called "Line Feed" (LF).

'\r' or '0x0D' (13 in decimal) -> This one is called "Carriage return" (CR). 

Different Operating Systems handle newlines in a different way. Here is a short list of the most common ones:

DOS and Windows:

They expect a newline to be the combination of two characters, namely '\r\n' (or 13 followed by 10).

Unix (and hence Linux as well):

Unix uses a single '\n' to indicate a new line.

Mac:

Macs use a single '\r'. 

ES_MULTILINE is an Edit Control Style. Since the default value is a single-line edit control, ES_MULTILINE specifies wheater or not the textbox shall have multiple lines or not. All values and the specifics about what ES_MULTILINE does in detail can be read here: https://learn.microsoft.com/en-us/windows/win32/controls/edit-control-styles