This is caused, as far as I can tell, by CVDisplayLink getting stuck.
Apple apparently are incapable of writing a simple timer robustly.
So if it remains stuck after a second delete and recreate it to force it
to restart.
Fixes#2016
The constants `NSControlStateValueOn`, `NSControlStateValueOff` and `NSControlStateValueMixed` are only available for macOS 10.13 or above according to https://developer.apple.com/documentation/appkit/nscontrolstatevalueon?language=objc.
Without this commit, compilation fails with this message:
```
glfw/cocoa_window.m:1537:28: error: use of undeclared identifier 'NSControlStateValueOn'
item.state = NSControlStateValueOn;
^
glfw/cocoa_window.m:1539:28: error: use of undeclared identifier 'NSControlStateValueMixed'
item.state = NSControlStateValueMixed;
^
glfw/cocoa_window.m:1542:47: error: use of undeclared identifier 'NSControlStateValueOn'
item.state = controller.isDesired ? NSControlStateValueOn : NSControlStateValueOff;
^
glfw/cocoa_window.m:1542:71: error: use of undeclared identifier 'NSControlStateValueOff'
item.state = controller.isDesired ? NSControlStateValueOn : NSControlStateValueOff;
^
```
To fix this, simply redefine the constants to use the old and now deprecated constants on older macOS versions.
The code that causes this was introduced in 98519bf326.
`glfwGetCocoaKeyEquivalent()` in `glfw/cocoa_window.m` expects the returned characters to be of type `unichar`, which won't work for all unicode characters because it is defined as `unsigned short` according to https://developer.apple.com/documentation/foundation/unichar?language=objc, which is only guaranteed to be at least 16 bits in size. The code calling this function also expects the encoding to be UTF-16.
When I added the various keys in https://github.com/kovidgoyal/kitty/pull/1928, I missed these facts. This means, that `glfwGetCocoaKeyEquivalent()` will behave unexpectedly when called with any of the new-ish keys. Luckily this function is currently only used for determining the macOS shortcut for `new_os_window` but I plan on using it more in the future.
Some of the constants, e.g. `NSBackspaceCharacter` are UTF-16 constants, so we can't just use UTF-8 everywhere.
I fixed the problem by using either UTF-8 characters packed into a `uint32_t` or UTF-16 characters in a `unichar` and then converting them to a UTF-8 encoded char string.
`NSEventModifierFlagNumericPad` isn't guaranteed to fit in a `unichar`, which made this undefined behaviour. It also didn't work. I tried to make it work using `NSEventModifierFlagNumericPad` as a modifier instead, as can be seen in this commit, but couldn't get it to work either because the constants used are native key codes and not unicode characters. Therefore the numpad keys will be removed in the next commit.
