splitwindow: Fix document selection popup on GTK >= 3.15.9 && <= 3.21.4

[b4n]

Works around GTK bug https://bugzilla.gnome.org/show_bug.cgi?id=769287.
Stop emission of the ::show-menu signal if it was first emitted from an
inactive toggle button, which happens only in the buggy case.
This workaround tries to not negatively affect a correctly behaving GTK
version in the unlikely case some distributions patched their version.

Fixes #1149.

[elextr]

Works for me, GTK 3.18.9 but with the messages

(geany:32192): Gtk-CRITICAL **: gtk_widget_unmap: assertion 'GTK_IS_WIDGET (widget)' failed

(geany:32192): Gtk-CRITICAL **: gtk_widget_unmap: assertion 'GTK_IS_WIDGET (widget)' failed

(geany:32192): Gtk-CRITICAL **: gtk_widget_unrealize: assertion 'GTK_IS_WIDGET (widget)' failed

(geany:32192): Gtk-CRITICAL **: gtk_widget_unrealize: assertion 'GTK_IS_WIDGET (widget)' failed

[b4n]

but with the messages […]

Ah right, that's Scintilla. I even had a fix somewhere, I'll find it back. But it's not so bad, it's "simply" that the widgets (scrollbars) in question are NULLs at this point, not plain invalid memory.

[b4n]

@elextr https://sourceforge.net/p/scintilla/bugs/1873/

[elextr]

Ok, hopefully that will be in Scintilla 3.7.1 which will probably not be in 1.29, so this should also wait.

[b4n]

@elextr why wait? It's totally unrelated to the fix, and it can happen a number of other ways. Also, the linked patch is totally trivial, it just guards 4 calls not to happen on NULL values, I'm confident you can review it for typos (like not guarding the right variable or something) in 5 minutes and we'll be sure it won't be a problem backporting it.

[elextr]

Ok, if you want to do that.

[elextr]

Works for me 👍