It's obviously too wide for a transition. The best way to solve this is to fill it. You'll never match the edges/colour so don't try.

Personally, I would install a marble threshold or a [tile mosaic] (https://atlastile.com/wp-content/uploads/2020/01/tile-accent-border.jpg). That would let you use grout to hide the imperfect edges on the floor and get the height right.

The flooring company might make a threshold/transition piece that will be of the same style and color. They probably have their catalog online.

You might be able to use a joist hanger type thing. They're intended to work through a combination of through-bolting and bearing so that the wood would rest on a little ledge. This is a thing I've done in the past. It's pretty visible, but it'll hold the bed together. Home Depot carries things like this - by the hardware (nails/screws/etc). It's hard to tell what piece is broken, though.

I'm a bit eccentric. I would get colorful tile and make a mosaic in the gap. It would add a little pop!

Yeah, this is the downside of chipboard: it's only kinda-repairable at best.

Wood glue + pressure is a good first step. Wipe the edges carefully to minimize seepage out the edges. After it drys you can rub a dark brown crayon around the edges to get wax into any visible seams to hide them.

To reassemble, you'll want to consider if you can use something like T-nuts. Or, as the other poster suggested, something stronger behind that you actually screw into. Consider that if this part was this weak, probably so are any other similar attachment points. So maybe while you're at it you could reinforce other places similarly.

How close is your toilet to your shower? I’ve been on a few jobs where the toilet was not caulked, the shower spray migrated under the toilet, and leaked below.

I just can't visualize it... I need a picture from further away... Wood glue will hold the wood together but not provide much in strength... But and bolt option if possible or oversized washers if you here Bolting into the frame

Use a straight edge to continue the poorly cut piece's edge through onto the adjacent piece. The best way to do this is to put down a layer of making tape on the floor, and then a layer of making tape on a ruler or level.

Use glue (Elmer's, wood glue, hot glue, whatever, just NOT super glue or epoxy) on the tape to stick the straight edge down in position. Be really slow and careful to make sure it runs straight from the bad cut to the doorjam. Let it dry.

Then use a brand new box cutter to run along that edge and cut through the flooring piece. You'll probably need to snap the blade off every four or five passes. Go slowly so you don't walk the cut. Once the board is cut you can work a painter's scraper under the masking tape and get it off the floor clean, no mess. Now you have two straight edges on the gap that aren't square, but they're straight.

Then you make a template of the area with cardboard, because again it's not going to be square. Just tape a few pieces together to get your edges and things right.

Get yourself a transition from the big box store, or some other board that's a contrasting color. Whatever it is, it needs to be thinner than the surrounding floor by at least 1/16". Use your cardboard template to cut down your new board. If it's wood, buy yourself a little hand plane and use that to dial in toward the size of your template.

Then you use a thin layer of titebond adhesive on the floor and stick your new board down. Be gentle with pushing it down so that you make sure it goes even with your existing floor all the way around. Fill in perimeter gaps with black or grey caulk.

Then try not to look too closely at it ever again.

The best attempt on a repair would be to pick away any loose bits of particle board that prevent perfect mating, then lightly coat both the chipped pieces and the base with a good quality wood glue and mate them together. Wipe off any excess you see when you mate them together, and clamp down on them using a piece of scrap wood to ensure they would stay in place. If it were me, I'd probably use painters tape to ensure they remain in place before I clamped the wood down. Regarding particle board, it doesn't look any dryer than your typical press-wood furniture; the wood glue will do a good job of forming a good bond.
Like the other post says, you might need to get a little creative with the bolts; my bet given how this looks, you might need to secure the bolts and such with some JB Weld on top of doing the repair, joining both the bolts and whatever flanges that those are secured to.

I have faced several of these types of transitions. If you have some of the original flooring, you could try to meticulously trim/fit a plank and shim it to the level of the floor then secure it down with construction adhesive, but I've found trying to make it "invisible" is difficult, and matching the gap, with its imperfections, is a meticulous process.
Next best (if you're good at matching staining and mid-level woodworking) is to take a plank and cut it into a threshold that fits relatively tight to that gap with some overhang onto the flooring, stain/varnish, then secure it in place. Finally, you can look for a store-bought transition that would do the same function as a custom built one - it'll likely not match as well, but it would certainly fill the gap.

The wood will have very little strength to hold the screws afterwards, you will probably have to be creative with nuts and bolts to actually hold it together afterwards. Or longer screws and a 2x4 in the back.

Any reason why you don't want to pit a threshold there? That's kinda what they are for.

If it’s a shower only issue, it sounds like it might be the tub spout. The diverter might not be closing fully and this could se sending the water back into the spout which could find it’s way back into the wall. I came across a product recently that’s called red disc seal or something and this might be a cheap attempt to fix the problem. My spout is kohler and it only has a set screw and it allows water to travel back down the copper pipe into the wall. I’ve been chatting with customer support a lot recently

They're talking about the trap. There are various designs, but yes the vent/overflow is a part of that which allows water to flow without creating a vacuum. It wouldn't make sense for it to be an issue if it only happens during showers.

Try running a shower with nobody in it. Does the shower head or the tub spout leak or spray some back onto the wall? If not, check the leak - it's probably dry. If it's dry, you need to check the seal around the vent/overflow, and around each handle and spout. You might need to tighten the screws a little on the vent, and caulk around it along the top half.

If the shower isn't spraying onto the wall with the handles, and you still see leaking, then you've got a leak somewhere between your tub spout and the shower head. Hopefully it's just the shower spigot in that case, because you can unscrew the entire thing, put some putty or Teflon tape on it, and screw it back in.

Another option would be to pour a bucket of water against that wall of the shower and see if that leaks. The biggest thing is just finding whether it's the plumbing leaking or the wall.

Hi, I had a leak in my shower too. I don't know if you have the same issue, but this is what I found. Because new tubs are fiberglass, they move too much. This caused the caulking at the top of the tub section to separate. Not enough to see, but just a hairline separation. Whenever I took a shower the water would get pulled into the caulk line and follow it to the edge of the tub and down the side. It was invisible. It then went behind the baseboard and leaked to the floor below. Took me a while to figure that one out.

Good luck.

If it was me I would try to fit a piece in that gap and make it seamless. Using some filler compound or something to fill in any small gaps. Putting a threshold there would always feel like a band aid to me.

Check your lumberyards for sill plates and thresholds.

I had the same issue. It turns out that the o-ring in the spout was bad. Basically when you use the diverter the water leaked behind the shower since the o-ring was bad. You could try taking the spout off and see if that o-ring is still intact.

Haha actually take what I wrote here back about the drain and focus on the shower arm i thought it was the baths causing the problem not the wife's shower at first read. But the idea still helps eliminate possibilities before you go cut some holes. Fill and drain sink, toilet and tub to rule those out. You've got a drop ear at the shower.

First I would reapply plumbers tape and maybe a little pipe dope and reattach the the shower arm. If that doesn't fix it odds are the supply leak is the connect from your shower pipe to that drop ear. That will either be soldered for copper or some sort of pex fitting and you'll have to cut drywall to check.

For drain issues: Fill the tub, tint the water and drain it. Look for that color. Sounds like a waste side leak. I had this problem last year. Bad fitting going from cast iron to PVC and it broke the seal over time. Be glad it's not your kitchen ceiling like mine was.

We have a glass enclosed shower in our master, that sits on tile, had a similar problem. No issue when the rest of the family used the shower, problem when my wife did. Turned out she would let water spray from the second shower head at the shower ledge while shaving, not realizing this would drip down the glass and eventually leak outside of the shower. It wouldn't immediately drip below the shower, and by the time the water leaked down, the surrounding tile was dry. Simple fix - dont let significant water be blasted at or sit at the bottom of the glass surround.

How wet is the wall getting between the shower head and the tub?

Fill it in with fixall and then custom cut a piece of 1/4 inch thick oak trim to cover that area.

Get some molding for the transition between rooms? You could cover the poor cut with that

Yeah that was my first thought too, but ours isn’t tile and grout. I honestly wasn’t sure what it was at first, as I hadn’t seen another shower/tub like this despite all the houses we looked it, but to me it just looks like “cheap crap.” After poking around, I think it’s a “glue up” shower. It feels and looks like really thin overlapping sheets of vinyl/plastic/whatever. Which does feel like that’s more seams where a leak could originate, but despite that I still think it’s plumbing related and either at the handle or further up at the shower head. I’ll plan to tear into it again tonight after putting the kids to bed.

If it's not leaking after a bath but it is after a shower then you need to see where the water is going when running the shower. Water at the shower head, water on the walls. Even outside of the tub. If your wife is not properly using the shower liner to prevent water from spilling outside of the tub, the water could be intruding through the bathroom floor most likely where the tub meets the flooring.