bug#47581: 27.1; tab-bar missed mouse clicks on MS-Windows

[Ioannis Kappas]

There appears to be an issue with the tab bar on MS-Windows that
sometimes it does not register mouse clicks.

Steps to reproduce

1. Run emacs -Q.
2. M-x tab-bar-mode, the tab bar opens with a tab on the *scratch*
   buffer.
3. Press the + button on the tab bar, a second tab is created on the
   same buffer.
4. Keep switching between the two tabs using the mouse, you will
   notice that sometimes clicking on the inactive tab does not
   activate it as expected.

Analysis and likely solution to follow.

In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-11-19 built on fv-az68-340
Repository revision: ec297125a76481c55390d0b329e541907879d6f3
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 10

Configured using:
 'configure --prefix=/mingw64 --build=x86_64-w64-mingw32 --with-modules
 --without-dbus --without-compress-install 'CFLAGS=-march=x86-64
 -mtune=generic -O2 -pipe' CPPFLAGS=-D__USE_MINGW_ANSI_STDIO=1
 'LDFLAGS=-pipe
 -Wl,--dynamicbase,--high-entropy-va,--nxcompat,--default-image-base-high''