I found a segfault in your last release of Geany (1.23.1). When I writed over 1 800 000 000 lines, Geany was K.O ... I haven't the real number (copy -> paste -> "boum"), but it's probably an INT size (2 147 483 647).
Yes, line numbers are limited to maximum signed int, usually 2^31-1 on 32 bit systems.
Taking a line from the C language spec, what happens when you reach that limit is "undefined".
But it is also possible that you have reached a memory limit if on a 32 bit system.
Not just line numbers even buffer size and offsets, Scintilla uses int all over the place where it should use more appropriate types (eg. long, unsigned long, size_t, intptr_t, ptrdiff_t, etc)