Looks like this is caused by scrollbar-gutter: stable.

Note that we disable clipping in CSS in this widget:

.ve-ui-completionWidget .oo-ui-popupWidget-body {
	/* The contained MenuSelectWidget is already clipped (T330401) */
	/* TODO: This could be done in JS if OO.ui.ClippableElement provided a way to disable clipping */
	overflow: hidden !important; /* stylelint-disable-line declaration-no-important */
}

Change 975063 had a related patch set uploaded (by Esanders; author: Esanders):

[VisualEditor/VisualEditor@master] CompletionWidget: Fix scrollable-gutter in CSS-disabled clippable element

https://gerrit.wikimedia.org/r/975063

Change 975063 merged by jenkins-bot:

[VisualEditor/VisualEditor@master] CompletionWidget: Fix scrollable-gutter in CSS-disabled clippable element

https://gerrit.wikimedia.org/r/975063

I've seen this also affect notifications as well as they are loading.

(Filed T351481 for that)

Change 975309 had a related patch set uploaded (by Esanders; author: Esanders):

[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (5b5119dd8)

https://gerrit.wikimedia.org/r/975309

Change 975309 merged by jenkins-bot:

[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (5b5119dd8)

https://gerrit.wikimedia.org/r/975309

Tested on desktop and mobile(bug irreproducible)

Before:

After: