Fix cursor stuck on HSplit in Advanced Importer

[SaracenOne]

Very minor cosmetic bug fix.
Closes #84002

[YuriSizov]

Doesn't look like a minor or cosmetic bug. Seems like a bug with input handling in subviewports. I'm not sure this is a good solution, or indeed a good workaround.

[SaracenOne]

Hmm...I can try and investigate this a bit deeper later on. The root cause seems to be that SubViewportContainer has special handling in _gui_input_event where it doesn't update the cursor if it's over one. I assume what it's meant to do is derive a cursor from the contents of the viewport, but since the viewport doesn't have an Control elements inside it, it may not be updating:

		if (DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_CURSOR_SHAPE) && !Object::cast_to<SubViewportContainer>(over)) {
			DisplayServer::get_singleton()->cursor_set_shape(ds_cursor_shape);
		}

[akien-mga]

CC @Sauermann

[akien-mga]

Superseded by #84661.