Regardless of how low you change the zoom while in-game, the skill panel will never zoom smaller than the zoom setting you have configured in the options panel. However, if you were to zoom to a size small enough, on a narrow enough level, that the level area is less wide than the skill panel, it becomes impossible to click a section of the skill panel that extends beyond the width of the level area, as the mouse won't move far enough.
In windowed mode, there is the workaround of using the "Release Mouse" hotkey. In fullscreen mode, the only workaround is to zoom in enough that the displayed level area's width is equal to or greater than that of the skill panel.
This is already fixed for the next update (and the fix will be included in the next experimental), but making the topic so that people are aware this is a known issue.
