I've found that tabindex behaves unpredictably if you click on a control, but only in some circumstances, while in other cases clicking on a control doesn't affect tabindex at all. It also seems to matter what type of control it is. For example, in the attached testcase (run on Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a3pre) Gecko/20070219 Minefield/3.0a3pre), I have 6 controls, all with a unique tabindex between 1 and 6. I find that if I tab to a pushbutton with a low tabindex, then click on a pushbutton with a higher tabindex and then hit the 'tab' key, the focus goes back to the previous pushbutton that had focus prior to the click. So if I tab to a pushbutton with a tabindex="2", click on a pushbutton with tabindex="4" and then hit the tab key, the pushbutton with tabindex="2" will get the focus. However, if I tab to an input textfield with tabindex="3", click on the pushbutton with tabindex="4", and then hit the tab key, the focus will go back to the pushbutton with tabindex="1". So even the bad behavior isn't consistent. This may be related to bug 257687, but my testcase doesn't involve JS, so I'm not sure that it is.
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