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Currently in Dolphin, when you are in group mode it is easy to select all the files in a group, you just click on the header. It is also easy to select individual files even with one-click activation using the + button on the files. However, there the only ways to select every file in the folder is to use the keyboard (usually ctrl+a), add a button to the toolbar (which can make it very crowded), or drag a rubber band selection (which is infeasible for large directories).

My idea is that, at least when you are in group mode, there is a single group heading for "All Files". There would be no files in the group, but if you click on it it will select all the files in the folder. Clicking on it again would deselect them. It could either be at the top or the bottom of the screen, both have advantages and disadvantages so I am not sure which is better. It might even be something each user can set. Also, at least as an option, I think clicking on that group when files are already selected would invert your selection. Alternatively, there could be a small button next to the "All Files" group name to let you do that (the area next to group names is pretty much wasted space right now).