If I have ten text files in a folder and I drag the mouse over five of them, they are all selected. That's fine.
Is there some way to undo all at once what I just did instead of clicking one each file individually, or clicking another folder then coming back to this folder?
[ul][li]If they're the only things selected and you're asking how to deselect everything:
You can make a toolbar/menu button or hotkey which runs Select NONE to deselect everything. (There isn't one by default since you'd normally use the mouse, as described next.) CORRECTION: As Jon points out, there's already a button which does Select NONE on the default toolbar, 4th from the left.
[ul][li]If you're any display mode other than Power Mode:
If they're the only things selected and you want to deselect everything then you can simply click on some space in the file display that isn't a filename to clear the selection. That works in the same way as Explorer.
Or you can click the individual filenames with the middle mouse button (if you have one and it's not configured to do something else in your mouse drivers) to toggle them, which is the same as holding ctrl and clicking with the left button.
Or you can drag a rectangle around the files you want to deselect using either ctrl-left or just the middle button. (Start dragging in an empty space, not on part of a filename, else you'll drag the files instead.) That toggles the selection of the items that you draw the box around.
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[li]If you are using Power Mode:
Then it depends a bit on how your Power Mode mouse buttons are configured but normally you would just drag over the files again to toggle their selection the same way they were selected to begin with.
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[li]If you have other files selected and want to deselect only the .txt files then you can:
[ul][li]Use Edit -> Select... in the default menus, click Advanced (if it opens in simple mode), change the Action to Deselect matching items, and set the filter to Name - Match - *.txt, Use Wildcards.
or
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[li]Create a toolbar/menu item or hotkey which runs this command:
[b]Select DESELECT ".{dlgstring|Enter extension to deselect}"[/b]
Then run it and type txt into the window that appears to deselect all *.txt files.[/li][/ul][/li][/ul]
...There are a bunch of other ways you could do it if none of the ones I've listed so far are convenient but hopefully one of those will do the job.
If you're in Details, List, Icon, Thumbnails or Tile mode, just click somewhere outside a filename, and anything selected will be deselected.
If you're in Power mode, file selection is persistent, to to deselect them just repeat whatever you did to select them in the first place and it will toggle the state.
To deselect ALL files you can use the Deselect command which by default is the right-button action on the fourth toolbar button from the left (hover over the buttons with the mouse to see a tooltip pop-up showing the commands).
Belated thanks to Leo and Jon. Thanks very much for your help. What a fantastic program Opus is. I wish I knew it as well as you two, but that day will come.