drawing 185 lines or more freezes up under certain conditions

[Moth-Tolias]

hello. i'm having an extremely strange issue. the following code freezes up when drawing 185 lines or more; it doesn't crash, it just stops updating or responding to input, and i have to kill it from the console with ctrl + c.

import std.stdio;
import bindbc.sdl;

void main()
{
	SDL_Window* window;
	SDL_Renderer* renderer;
	bool isRunning = true;

	loadSDL();
	SDL_Init(SDL_INIT_VIDEO);

	enum windowSizeX = 512;
	enum windowSizeY = 288;
	enum windowTitle = "line test";

	window = createWindow(windowSizeX, windowSizeY, windowTitle);
	renderer = createRenderer(window);

	int lines = 0;

	while(isRunning)
	{
		SDL_Event currentSDLEvent;
		while (SDL_PollEvent(&currentSDLEvent) != 0)
		{
			if (currentSDLEvent.type == SDL_QUIT)
			{
				isRunning = false;

				import core.stdc.stdio: printf;
				printf("quitting...\n");
			}
		}

		SDL_SetRenderDrawColor(renderer, 0x00, 0x00, 0x00, 0xFF);
		SDL_RenderClear(renderer); //required

		SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);
		drawLines(renderer, lines);

		SDL_RenderPresent(renderer); //required
	}
}

SDL_Window* createWindow(int windowSizeX, int windowSizeY, string windowTitle)
{
	import std.string: toStringz;
	enum windowFlags = SDL_WINDOW_SHOWN;
	SDL_Window* result = SDL_CreateWindow(
		toStringz(windowTitle),
		SDL_WINDOWPOS_UNDEFINED, // initial x position
		SDL_WINDOWPOS_UNDEFINED, // initial y position
		windowSizeX,
		windowSizeY,
		windowFlags
	);

	return result;
}

SDL_Renderer* createRenderer(SDL_Window* window)
{
	enum rendererFlags = SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC;
	enum SDL_firstAvailableDriver = -1;
	SDL_Renderer* result = SDL_CreateRenderer(window, SDL_firstAvailableDriver, rendererFlags);
	return result;
}


void drawLines(SDL_Renderer* renderer, ref int noOfLines)
{
	foreach(line; 0 .. noOfLines += 1) // += is required?? ++ seemingly works fine
	{
		immutable int x = (line % 100) * 3;
		immutable int y = (line / 100) * 3;

		import std.math;
		immutable wtf = cast(int) (sin(cast(float)line) * 4); // different multipliers have different results, eg:
		// 1 seemingly has no limit
		// 2 halts at 449
		// 3 halts at 371
		// 4, 5, 6, 7 halt at 185
		// 8, 9 halts at 356
		// 10 halts at 497

		SDL_RenderDrawLine(renderer, x, y, x + 1 + wtf, y + 1); // the + 1s are required???
	}

	import std.stdio: writeln;
	writeln(noOfLines);
}

i haven't the foggiest idea why this would happen, least of all why += should be different from ++ - nor why the +1s seem to be required.

using version 1.5.0, dmd, windows 10.