<p>I think you should install the <code>-dev</code> package for GTK+3. On my more recent Ubuntu version the package is called <code>libgtk-3-dev</code> but IIRC it used to be called something else. It's the GTK+3 version of the <code>libgtk2.0-dev</code> one you list above.</p>
<p>When compiling with the official tarball, pass <code>--enable-gtk3</code> to the <code>configure</code> script so it will fail and tell you whether you have the GTK+3 stuff installed.</p>
<p>Most likely when Ubuntu 16.04 was released, the Geany packages were built with GTK+2 so using commands such as <code>apt-get build-dep geany</code> probably won't install the GTK+3 stuff you need.</p>

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