When we encounter clipboard input that cannot be mapped to a keyboard
key, we try our best to map it to an event. If if is an alphanumeric
character or a wide glyph (per the old width algorithm), we will pass it
through as the UnicodeChar of a key event. If it's *not* wide and *not*
alphanumeric, we will synthesize a set of Alt+NumPad events.
Those events comprise a set containing...
1. Alt Down (modifiers = LAlt, UnicodeChar = 0)
2. Numpad 0 Down/Up (modifiers = LAlt, UnicodeChar = 0)
3. Numpad 1 Down/Up (modifiers = LAlt, UnicodeChar = 0)
4. Numpad 2 Down/Up (modifiers = LAlt, UnicodeChar = 0)
5. Alt Up (modifiers = 0, UnicodeChar = [THE CHARACTER])
Because of event group 5, application developers seem to have taken a
dependency on receiving Alt Up + Character and don't seek to recompose
the original character from its numpad representation.
In pull request GH-8035 (!5394370), we stripped the old width algorithm
out and replaced it with a lookup table (finally!). Unfortunately, that
broke clipboard input for Chinese text as it was no longer considered
"Wide" for the purposes of detecting whether we should use numpad
events.
This commit introduces a version of GetQuickCharWidth that fulfills the
exact contract CharToKeyEvents needs, and doesn't answer for a codepoint
more. We'll use it in Windows to fix MSFT-32901370.
The Terminal analogue of this bug, GH-9052, is fixed by *never emitting
numpad events again.* We think this is okay because it looks like nobody
was ever handling numpad events... and that we were only using them as a
way to communicate within conhost (which we're *also* not using) and any
public exposition of event 5 as a contract was unintended.
VALIDATION
----------
I took this new implementation (with an early return) and the old
implementation and compared whether they would yield a numpad event or a
key event over the entire supported codepoint space [0000..FFFF].
They matched.
Fixes MSFT-32901370
Fixes GH-9052
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