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This is very unfinished, do a simple container that animates
insertion and removal.

The resizing is too distracting otherwise, and reserving space
would be unnecessarily complicated.

When a needs-attention tab is scrolled out of view, set the
.needs-attention class on the corresponding overflow node,
so we can give it some styling.

Add the .needs-attention style class to tabs whose pages have
GtkStack::needs-attention set to TRUE.

Scroll the visible tab into view when it changes.

This is typical case in web browsers or editors.

Populate the example with typical web page titles, and move the
new tab button to the headerbar.

We were triggering warnings when a stack child goes away.

This gets us a bit closer to the desired user experience.

Cut down on padding, and make close buttons square.

Without scroll buttons, all the autoscroll machinery is no longer
needed.

No need to complicate things with options. For now, lets focus
on getting the scrolling right.

Without custom tab creation in GtkTabStrip, this no longer needs
to be public.

Allowing custom tab implementations ties our hands for what we can
do with the tabs, so lets figure out the desired user experience first.

This is distracting from getting the desired user experience.
It can be added later.

Arrange for autoscroll and keyboard scrolling to end on a
tab boundary.

Just so we can see it is there.

Instead, derive from GtkContainer and use a box gadget to
manage our internal children.

The selector was not specific enough, and the hover styling
was missing.

This broke when I introduced the scrolling. We need to
iterate over the children of the tabs box to update their
state.

Add edge properties on both GtkTabStrip and GtkTab, and make
GtkTabStrip update its orientation and scrolling direction when
the edge changes.

This makes testtabstrip use its own tab class, which puts
a close command in a popover, just to show that this is
possible.

Add a ::create-tab signal that can be used to override the default
choice of simple or closable tabs with a custom tab implementation.

Implement scrolling arrows. This is currently using a scrolled window.

Add a boolean property to instruct GtkTabStrip to use closable
tabs instead of simple tabs.

This adds a close button to the tab in addition to the label.

This is a very barebones subclass of GtkTab, which shows the page
title in a label.

This adds GtkTabStrip and GtkTab, which will eventually allow
creating typical notebook scenarios with GtkStack.

Inspired by the PnlTabStrip prototype that Christian Hergert
wrote for gnome-builder.

This used to live in gtknotebook.c. Since we may use it in
other places in the future, move it to gtkprivate.c where we
already have other shared accumulators.