The keyboard shortcut for resizing all columns in a report-mode (also known as Details mode) list view control to fit the current content width is Ctrl+Num+. That’s the + key on the numeric keypad. (If you’re using Explorer, you can also right-click the column header and choose Size All Columns to Fit.)
Note that this command is a verb, not a state, so it takes into account the contents of the listview at the time you press the hotkey. If the contents change and you want the columns resize based on the new contents, you’ll have to press the hotkey again.