fix[devtools/tree/element]: onClick -> onMouseDown to handle first click correctly

[hoxyq]

There is a weird behaviour in all shells of RDT: when user opens Components tab and scrolls down a tree (without any prior click or focus event), and then clicks on some element, the click event will not be fired. Because click event hasn't been fired, the focus event is fired for the whole list and we pre-select the first (root) element in the tree:

react/packages/react-devtools-shared/src/devtools/views/Components/Tree.js

Lines 217 to 226 in 034130c

	const handleFocus = useCallback(() => {
	setTreeFocused(true);
	if (selectedElementIndex === null && numElements > 0) {
	dispatch({
	type: 'SELECT_ELEMENT_AT_INDEX',
	payload: 0,
	});
	}
	}, [dispatch, numElements, selectedElementIndex]);

Check the demo (before) what is happening. I don't know exactly why click event is not fired there, but it only happens:

1. For elements, which were not previously rendered (for virtualization purposes).
2. When HTML-element (div), which represents the container for the tree was not focused previously.

Unlike the click event, the mousedown event is fired consistently.

Before

before.mov

After

after1.mov

Tested that it works in all shells, including the select / deselect features (with metaKey param in event).

[motiz88]

lgtm!