Cannot tab between block inputs in the palette

[cwillisf]

Expected Behavior

When a block has multiple input fields, pressing the tab key should move from one input to the next

Actual Behavior

The tab key appears to do nothing when the cursor is in an input field in a block in the palette. It works as expected in the regular workspace.

Steps to Reproduce

1. Find the "go to x: {} y: {}" block in the blocks palette
2. Click into the "x" field
3. Press tab

System Details

Observed in both Chrome and Firefox. Behavior might be slightly worse in Firefox..?

[cwillisf]

The cleanest fix to this appears to be upstream, in the getNextSibling() method in Blockly's FlyoutNavigationPolicy. Something like this:

-    if (index === -1) return null;
+    if (index === -1) return this.policy.getNextSibling(current);

(and similar for getPreviousSibling)

[rachel-fenichel]

We chose to not implement this behaviour in keyboard navigation because it made all basic navigation more complicated than we wanted: RaspberryPiFoundation/blockly-keyboard-experimentation#320 (comment)