Change MOUSE_SELECTION_KEY to Fn + cmd

[Khushiyant]

What does this PR do?

MOUSE_SELECTION_KEY is changed to Fn + cmd in MacOS platform

Testing

• Manually
• Passed linting & tests (each commit)

Commit changes

- MOUSE_SELECTION_KEY = "Fn + Alt" if PLATFORM == "MacOS" else "Shift"
+ MOUSE_SELECTION_KEY = "Fn + cmd" if PLATFORM == "MacOS" else "Shift"

Visual changes

Before

After

Changed to Fn+cmd is used to avoid confusion due to Mac keyboard layout

[Khushiyant]

@neiljp Please check

[neiljp]

I don't use a mac so this is difficult for me to confirm personally.

Which terminal emulators have you tested this on?

My understanding is that depending on the terminal emulator, eg. Terminal (built-in) or iTerm (app that some use), Alt/cmd may work differently, see for example #680 (comment)

Are you about on chat.zulip.org?

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If this does resolve this one particular case across terminals, more generally I'd like to combine this fix with a translation of the Meta which is necessary for urwid to understand, into something like physical_key_representing_meta() for user output (in platform_code.py), since Meta is not an available physical key in most cases.

Note that resolving the Mac (and cross-platform) situation properly would involve documenting the situation via #873, as well as ensuring we address points that have come up in topics that have come up in

• https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Alt.20.2F.20Meta.20keys
• https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/About.20Popup.20.28alt.2Fmeta.29

[Khushiyant]

@neiljp

• I have used the default zsh terminal for testing
• I feel that key mapping for above problem is correct but instruction text is displayed wrong according to Mac keyboard layout since there is no Alt in it.

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In case of documentation, I'll change the things accordingly

[neiljp]

@Khushiyant I was referring to the application you're using to run zsh (which itself is a shell), so that doesn't quite answer my question.

[Khushiyant]

@neiljp I have tested the change on Terminal(built-in, ver 2.12.7) and also on iTerm