The user can select buttons in the HTML5 toolbar. This looks ugly and confusing at times.
This css property can be set on elements: -webkit-user-select: none; -khtml-user-select: none; -moz-user-select: none; -ms-user-select: none; -o-user-select: none; user-select: none; We have to be careful when using this though since there are some items in the UI which we do want to be selectable, like the text in the clipboard panel for example.
The problem was likely related to the toolbar handle. The user is meant to be able to click and drag the handle and might slightly miss it when doing so, causing unintended selections. The issue was more prominent on touch devices, since a finger is less precise than a mouse cursor. This was fixed in commits r39184 & r39165 for bug 7908. User selection is now disabled globally in the toolbar. Text in the clipboard textarea is still selectable. Note that it is debatable if this was the correct decision, since there might be users expecting selection to work here. However, in the current state, the toolbar feels more like a GUI element than something on a web page.
