SUMMARY The history is currently a simple list view with text strings. To pick up an older calculation, you have to: 1. Select the text with the cursor 2. Right click and choose Copy 3. Right click into the input field and choose paste This is obviously a very bad UX. Keep in mind that in case of 3. there even exists an annoying bug: the paste entry will become grayed out, when you re-focus KCalc - I will open another bug report for that. SpeedCrunch does this much better: Here you have a list view too, but the items are not select-able text, but are pasted automatically into the input field on double click!
See https://invent.kde.org/utilities/kcalc/-/issues/20
A possibly relevant merge request was started @ https://invent.kde.org/utilities/kcalc/-/merge_requests/188