Resizing Mattermost sends it crazy

[AlanGriffiths]

1. Open the mattermost-desktop snap on a Mir compositor (without hacking it to use Wayland)
2. Try to resize it's window

Expect: window resizes
Actual: window disappears and mattermost hogs a CPU

(This doesn't occur using Wayland, so is likely to do with our Xwayland interaction)

[mattkae]

Note that this even happens when we fullscreen the mattermost application

[mattkae]

Fun fact: It does not disappear: it just gets a teeny tiny width and height. The very tiny black bar on the left hand side of this image is mattermost:

[mattkae]

The problem is that the WindowInfo::max_width/max_height are both 1. Time to figure out why 🥾

UPDATE:
The offending lines are:

#define SCALE_SIZE(type, prop) \
    if (mods.prop) \
    { \
        mods.prop = std::max(geom::type{1}, mods.prop.value() * inv_scale); \
    }
...
SCALE_SIZE(Width, max_width);
SCALE_SIZE(Height, max_height);

in XWaylandSurface::prep_surface_spec. max_width and max_height are both set to 32bit int max here

UPDATE, UPDATE:
That's a fun one. So what's going on here (AFAICT)? Let's take max_width as the example:

• max_width is type Width<int> which is just a Value<int> under the hood
• It is asked to be scaled by inv_scale, which is a float set to 1.f, which makes the value resulting from the multiplication be 2.14748365e+09
• This number then gets cast back to an int because the multiplication wants to return the initial int type, which results in -2147483648, which is obviously less than 1.

TLDR; this is happening:

#include <iostream>
#include <algorithm>

int main()
{
    int x = 2147483647;
    int y = static_cast<int>(x * 1.f);
    int z = std::max(1, y);
    std::cout << y << ", " << z << std::endl;
    return 0;
}

• y is -2147483648,
• z is 1

[mattkae]

Fixed by #3050