GameMaker Studio 2 Click-Drag Tile Selection - multipleselection

Question

I would like to implement a simple way to select a section of a tilemap, though I'm stuck on the math of updating the selected area as you drag.
I spent about 12 hours reasearching and trying different things to see if I can get this to work correctly. I didn't think this would be as tough as it is.
My prefered method of implementing the selection, is using four variables:
selection_x - X position of the top left corner of the selection
selection_y - Y position of the top left corner of the selection
selection_w - Width of the selection
selection_h - Height of the selection
The scrips tilex() and tiley() return the tile X/Y where the mouse is, calculating for zoom, and view offset.
Here is my attempt:
if(mouse_check_button_pressed(mb_left)) {
selecting = true;
selection_start = [tilex(), tiley()];
selection_stop = [tilex() + 1, tiley() + 1];
selection_x = tilex();
selection_y = tiley();
selection_w = 0;
selection_h = 0;
} else if(mouse_check_button(mb_left)) {
var old = selection_x;
selection_x = tilex() >= selection_x + selection_w ? tilex() : selection_x;
selection_w = tilex() >= selection_x + selection_w ? tilex() - selection_x : (old - selection_x);
var old = selection_y;
selection_y = tiley() >= selection_y + selection_h ? tiley() : selection_y;
selection_h = tiley() >= selection_y + selection_h ? tiley() - selection_y : (old - selection_y);
/*
if(selection_x >= tilex()) {
var old = selection_x;
selection_x = tilex();
selection_w += old - selection_x ;
selection_stop[0] = tilex() + 1;
show_debug_message("x 0");
} else if(selection_x + selection_w <= tilex()) {
selection_w = tilex() - selection_x + 1;
selection_start[0] = tilex();
show_debug_message("x 1");
}
if(selection_y >= tiley()) {
var old = selection_y;
selection_y = tiley();
selection_h += old - selection_y;;
selection_stop[1] = tiley() + 1;
show_debug_message("y 0");
} else if(selection_y + selection_h <= tiley()) {
selection_h = tiley() - selection_y + 1;
selection_start[1] = tiley();
show_debug_message("y 1");
}
*/
/*
if(selection_x > tilex() && selection_y > tiley()) {
} else if(selection_x < tilex() && selection_y < tiley()) {
selection_x = tilex();
selection_y = tiley();
selection_w = selection_x - tilex();
selection_h = selection_y - tiley();
}
*/
/*
if(selection_start[0] > selection_stop[0] || selection_start[1] < selection_stop[1]) {
//selection_start = [tilex(), tiley()];
} else {
selection_stop = [tilex() + 1, tiley() + 1];
}
*/
}
Thanks for reading! Hope this can get resolved soon.
- Searous <3