My hope is that during the process of development and testing of Tcl 8.6 (and you ARE testing your code against the alpha releases, right?), that developers will add information here regarding changes that either are required, or perhaps are recommended, for Tcl or C code.
MG The text widget's default bindings seem to have changed pretty considerably - a lot of things are now handled via custom events instead of direct bindings (notably things which move the cursor and alter selection). For instance, "Select all text" is now <<SelectAll>> instead of a direct binding on <Control-/>. Also, on Windows, it seems that <Control-a> is also linked to <<SelectAll>> by default. So, if you have custom bindings to alter or add to any of those in your app, you may need to adjust them for 8.6.