bug#41343: tab-bar-mode: Close tab on mouse-2 click

[Juri Linkov]

>> It's too late to change how keys are implemented for tab-bar.
>> The tab-bar keymap contains such keys as 'tab-1' and 'C-tab-2',
>> and clicking mouse-1 on tabs emits such events.  It doesn't emit
>> 'mouse-1' and 'mouse-2'.
>
> How do we translate mouse clicks into the likes of C-tab-2?

When mouse-1 is clicked on the close button, then the
Control modifier 'C-' is added to the menu-item symbol tab-2
emitted as an event.

> And what prevents us from emitting something like tab-close-2 when
> mouse-2 is clicked?

Then mouse-2 is still hard-coded and tab-close-2 is not much different
from the previous patch that emits C-tab-2.

Or in "tab-close-2" what does "2" mean: the button number 2 (mouse-2),
or the tab number 2 (tab-2)?

Should they then cover all combinations?  tab-1-mouse-1,
tab-1-mouse-2, tab-2-mouse-1, tab-2-mouse-2, ...

>> It should be possible to implement normal key bindings mouse-1/mouse-2,
>> but such change will not be backward-compatible.
>
> In what way will it be incompatible?

Currently tab-bar-make-keymap returns a keymap with menu-items.
This is like menu-bar is implemented.  The menu-bar displays
menu-items, and clicking on them emits events with their symbols.
The tool-bar is implemented the same way: it displays
menu-items, and clicking emits their symbols.
The tab-bar is exactly the same: displays menu-items
on the tab-bar, and clicking mouse-1 emits events
with tab names, e.g. tab-2.

Users already are using such add-advice that expect
tab-bar-make-keymap returning a keymap with menu-items.