When the "new user experience" tips are displayed, the tips Close button tooltip is partially hidden in some tips. To reproduce, for example in Chrome:

• go to the Add New post page
• open the Chrome dev tools > Application > Storage > Local storage, and delete the entry "GUTENBERG_NUX_1" if any Edit: the storage mechanism has changed, you need now to clear the whole Gutenberg storage entry
• refresh the page
• the first tip should appear
• tab to or mouse over the Close button
• the button tooltip appears and it's fully visible:

• click on "See next tip" to go to the following tips
• tab to or mouse over the Close button
• the tooltip is partially hidden behind the tip
• worth also noting the tooltip appears in the same place where the "pulsing dot" is

Worth noting the tooltip position is dynamically calculated to stay within the viewport, and that seems to work correctly. However, seems there's something wrong with the z-index. Not sure why when the popover takes the is-bottom class it gets a lower z-index.

.components-popover:not(.is-mobile).is-bottom {
    top: 100%;
    z-index: 99990;
}

However, I guess the whole position of the tip should be reviewed, because the tooltip position conflicts with the pulsing dot.