ledow opened the ticket and wrote:

Occasionally, I still run into these assertions:

assert(bottom > 0)
assert(right > 0)

which are always fatal and crash the game. It appears to be related to slow computers (ones that pause while autosaving for instance) and the mouse. A lot of the time on these sorts of machines, the mouse will stop updating while the save occurs and then have one large update once it returns - this could be the cause. This is usually when this assertion occurs and it can happen while dragging a window, for instance. I've had it happen on a Linux PC with a PS/2 mouse and on a Windows laptop with a trackpad or USB mouse (both of a similar spec - 1GHz, 512Mb). No matter what I use, eventually I will get one of these assertions. Fortunately, reloading the last autosave is usually fine and it's rare enough that it's not too annoying.

I've had it happen in the latest SVN and in the latest stable for several months now. It's unpredictable and randomly reproducible and seems highly dependent on the computer that's being used (faster machines don't exhibit the problem, or at least not quite as badly).

The cause seems to be the calculations within the function GfxFillRect of gfx.c (this is the only place that such asserts occur and it has both of these asserts within close proximity).

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/176