Setting a planet as a "default" is impossible for the engine, as far as I know. I looked into it while making the mod because I didn't want people to click uninhabitable planets with the viewer open and see ugly stuff. Or use it to cheat (they can't, so it's fine).

The name of the planet in the top text changing would be a little annoying to fix, but I can definitely add a new "no planet selected" message for clarity - brilliant idea.

@Sildraug
Ahhh nice investigating! That makes perfect sense. What you're describing about the UI actually is how it all works for Stellaris. Whatever you're "targetting" is the current scope, and the UI panels can only retrieve data about that thing. It's just that, unlike the vanilla planet view, my custom GUI doesn't auto-close when you click away.

So anyway, while making the mod I used the console (or my planet view mod) to open the custom UI, both of which are instant and don't have that annoying clear time of 1 day in which I could end up closing the panel. I never thought about players doing that!

One, treat the original selected planet as a 'default', so in the event that no planet is selected in the outliner, the user can still select continents from the original planet. This would be consistent with the header for the panel.

Two, provide some kind of UI feedback to the player differentiating between 'no planet selected' and 'no continents to explore on target planet'.

It also probably would be a good idea, if possible, to see about making that text label showing which planet is being displayed dynamic, so it can be in sync with what the window is actually showing. If there isn't a clean / easy way to go about that within the event text, maybe having a text label within the body of the panel (like you have for the individual unexplored continents) would be a prudent option instead.

Given how large the base planet view window for UI Overhaul is, I was queuing up build orders in pause, closing the planet detail view, and then not seeing clearable deposits when I followed up by opening the fired exploration event. The confusion came into the picture because the generated panel view still indicates the continents intended to display are the continents for the planet through which the event for the view was originally fired, by naming the planet in the header.

Given that, if you're willing, I can make two possible recommendations--either one could be a worthwhile route:

Alright, so I did a bit more investigating, and I figured out what's going on:
It's not really a compatibility issue, it's a workflow / feedback issue. I was able to replicate it with and without UI Overhauls active. When clearing the 'explore' blocker on a planet with unexplored features, the exploration window will only populate with the information for a planet that has its detail panel open simultaneously. In fact, changing which planet is selected by clicking through the outliner while the event is open will change which features are shown as available for exploration.

This is neat functionality, from a UI perspective! The part that was introducing confusion is that the message for having no planet selected in the background was exactly the same as selecting a planet with no unexplored features.

Good news! Well, possibly more actionable news, anyway. From what I'm observing, running Mysterious Worlds solely with UI Overhaul Dynamic ( https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1623423360 , to be as specific as possible, and using just the launcher's mod manager to identify the modset) produces the behavior I mentioned observing, where clearing the Explore Continent blocker produces an event that claims the planet is already fully explored, regardless of the presence of Unexplored Continent features.

Running solely Mysterious Worlds, it does produce an event with the identified number of unexplored continents--I'll try incremental sets to see if I can pin down what seems to be disagreeing.

Very weird, I really have no idea why the GUI would show a planet as explored if it has unexplored continents on it!

Does it happen with JUST "mysterious worlds" installed?

They do indeed have unexplored continent features--five, in most cases. I noticed they had those present in combination with the 'explore planet' blocker in the planetary feature list. I did see that my capital also behaved the same way when clicking to the 'view planet' menu, but that makes a lot of sense, and seemed likely to be intended behavior.

@Sildraug
Does the planet you're trying to explore actually have "unexplored continent" deposits on it?

Is it your homeworld? Because that's already explored.

Is it earth or a relic world? Because those aren't explorable.

Basically, go into observer mode and look around the galaxy to see if you have any planets with actually "unexplored" deposits on them!

Hey, I've tried loading in this modset in the listed order--it sounds like a very promising mechanical set! But I'm noticing the planetary exploration event menu that pops up after clearing the 'explore planet' blocker is only showing a 'planet fully explored' display, rather than the region menu you show in the screenshots. I have no other mods active, and I'm seeing this both with planets terraformed in advance of colonization and planets colonized without terraformation. I'm not running any other mods currently, so I'm pretty sure it's not a compatibility problem, or at least not one I'm introducing--do you have any suggestions for what I might need to tweak with to get Exploration to cooperate?