Prevent backspace and delete functionality in the in-call dial pad. (#6522)

Just as one cannot do when there inputting number while
on a call on their phone, it doesn't make much sense to
be able to do so while on a call in Element. The DTMF
tone history should be preserved.

src/components/views/context_menus/DialpadContextMenu.tsx

@@ -49,6 +49,13 @@ export default class DialpadContextMenu extends React.Component<IProps, IState>
         this.props.onFinished();
     };
 
+    onKeyDown = (ev) => {
+        // Prevent Backspace and Delete keys from functioning in the entry field
+        if (ev.code === "Backspace" || ev.code === "Delete") {
+            ev.preventDefault();
+        }
+    };
+
     onChange = (ev) => {
         this.setState({ value: ev.target.value });
     };
@@ -64,6 +71,7 @@ export default class DialpadContextMenu extends React.Component<IProps, IState>
                         className="mx_DialPadContextMenu_dialled"
                         value={this.state.value}
                         autoFocus={true}
+                        onKeyDown={this.onKeyDown}
                         onChange={this.onChange}
                     />
                 </div>