Hi Susan,
Using the version of AR you indicated, I edit any customer, go to the Transactions tab, and click Both to show everything. From here, I only require a single click into the document number field - without a document type, which assumes searching for invoices - and can type the invoice number and then Enter to find it. For me, a double-click was not necessary.
As you indicated, you could also use the Alt-F shortcut to go directly to the Find document field without having to click in it. Hopefully that is a useful shortcut and since it works every time
I suspect that some window other than Adagio has the focus at that moment, and it takes one click to return focus to Adagio and the second click to put the cursor into that field. In any case, that it is intermittent indicates that something else is (sometimes) getting in the way of the expected behaviour.
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Regards,
Softrak Tech Support