ScrollBar resize can cause unintentional scroll

[moniika]

Describe the bug

The scrollbar resize call can unintentionally scroll.

To Reproduce

1. Add 4 blocks with space between them and prepare a 5th block that would fit in empty space.

Blockly.Xml.domToWorkspace(
    Blockly.Xml.textToDom(
       '<xml xmlns="https://developers.google.com/blockly/xml">' +
       '<block type="controls_if" x="88" y="88"></block>' +
        '<block type="controls_if" x="288" y="88"></block>' +
        '<block type="controls_if" x="88" y="238"></block>' +
        '<block type="controls_if" x="288" y="238"></block>' +
        '</xml>'),
    workspace);
var xmlDom = Blockly.Xml.textToDom(
    '<xml xmlns="https://developers.google.com/blockly/xml">' +
    '<block type="controls_if" x="188" y="163"></block> +
    '</xml>');

2. Set breakpoints at onScroll() call in Scrollbar.prototyle.resize and in WorkspaceSvg.setTopLevelWorkspaceMetrics_
3. Add block in the middle of the existing blocks, should not trigger any scrolling.

Blockly.Xml.domToWorkspace(xmlDom, workspace);

4. Step through breakpoints observing that hostmetrics.viewLeft is 0 in Scrollbar resize, but in setTopLevelWorkspaceMetrics_ this.scrollY is set to small but non-zero value, where we would expect zero.

Expected behavior

There shouldn't be scrolling.

Additional context

This can cause unwanted extra emission of viewport change events, once they are enabled.

[rachel-fenichel]

Low priority because most of the possible triggers are blocked by workspace.setResizeEnabled(false), such as during drags.