SVN commit 1218185 by aacid enabled the "continuous" selection of a tool by double-clicking on it, for example when using annotations. I wish there was a (configurable) hotkey for this, but I cannot find it. Use case: when using a graphic tablet to scribble on PDF documents, I keep an hand on the keyboard for switching between tools (eg. keyboard "2" for inline note, "3" for free-hand line, etc) and one on the stylus. But if I wish to keep one tool active, I have to grab the mouse or travel with the stylus to the toolbar button.
The new annotation toolbar has a “Keep Active” action, you can set the shortcut like any other KActionCollection action now via Settings -> Configure Shortcuts.