M: Okay, the Graphic Program GIMP is being a bitch, as far as I can tell, and I'm saying it that way because things are not working in a way that I can't seem to backtrace the cause for if there is a cause that I have control over. But as far as I know, what I did was I increased the canvas size, pasted another layer, and then tried to paint on the first layer to give a solid background of some kind behind the second layer on the area of the canvas that had expanded. GIMP won't let me paint or do any editing on the original layer outside of the pixels that are already defined. I've never had this happen. I don't even know where to start. If it gives you any clues, just to see if I could get some sort of reaction, when I go to the second layer and I try to erase, not even because I want to, but just to see if I can get any reaction, it's like it won't erase. None of it will.

G: That symptom has a very specific cause in GIMP — you’ve hit the layer boundary limit, not the canvas limit.

When you resize the canvas, GIMP doesn’t automatically resize the layer; each layer still has its own defined pixel boundaries. So what’s happening is:

You increased the canvas size (workspace got bigger).
But your layer didn’t expand with it.
You’re trying to paint or erase outside that layer’s own rectangle — and GIMP blocks it silently.

That’s why tools appear “dead” outside the original image area.

Here’s how to fix it:

🩹 Quick Fix

Select the layer you want to paint on.
Go to Layer → Layer to Image Size.
- This stretches that layer to match the full canvas.
Now painting and erasing should work everywhere.

🧭 Alternative Method (if you want finer control)

If you only want part of the new canvas filled, or want to control where the old layer extends:

Choose Layer → Resize Layer.
In the dialog, set Width and Height to match the canvas, or manually position the layer.
Click Center if needed.

🧠 Why it happens

Think of GIMP like Photoshop with locked “sheet” sizes per layer — the canvas is the table; the layer is the paper sitting on it. You just made the table bigger, but didn’t replace your smaller piece of paper. So GIMP won’t let you draw on the table itself.

If even erasing doesn’t work inside visible pixels of the top layer, then a second issue could be active:

Check Layer Lock icons: ensure “Lock pixels” and “Lock alpha” aren’t on.
Check Mode: make sure you’re not in Quick Mask mode (the red overlay).
Check Selection: press Ctrl+Shift+A to deselect all.

Want me to help you systematically check those next (in case it’s more than the layer boundary)?

G: Good question — and yeah, it’s box-shaped.

Each layer in GIMP is defined as a rectangular bitmap — no exceptions. Even if the layer’s visible content is irregular (a cutout, a pasted shape, or transparent areas), the layer boundary itself is always a rectangular grid of pixels with fixed width and height.

So the behavior you hit happens in any of these cases:

You paste something and it creates a floating selection or new layer that’s only as large as the pasted content — that layer’s rectangle stops exactly at the edges of those pixels.
You enlarge the canvas, but don’t resize the layer — you now have extra canvas space but the layer’s pixel box didn’t grow.
You try to paint outside that box — nothing happens, because there is no pixel data there.

It doesn’t matter what the actual shape of your cutout is — GIMP still wraps it in an invisible rectangular boundary.

If you ever want to see where those limits are, choose:

View → Show Layer Boundary

You’ll get a yellow dashed rectangle showing the box edge. You can only paint inside that rectangle until you resize or expand the layer.

M: Okay, now try and diagnose the part about why the second layer doesn't seem to want to erase. There was a subset of things that happened, so it's not just what I had up there. I was able to work around the problem because I changed the pasted layer from normal to multiply mode, which allowed me to do what I needed. But it's a problem that could come up again. Okay, so when I pasted on the second layer, I adjusted it to where I wanted and some of its edge pixels were overlapping the background, so I told it to erase. It erased to the background color, and it's not because I had anti-erase or anti-transparency. What the hell is the name of it? Yeah, it's not because I had anti-erase on. I just checked the words. So I looked online quickly, and it said to add an alpha layer or alpha channel to the layer, which I did. Now the eraser has no effect at all. It doesn't even erase on that layer. Like right now, I am making the mouse motions that should completely obliterate the layer that I added. Instead, the cursor is just moving around. Opacity is at 100%. And like I said, when there was no alpha channel, it basically functioned like a paintbrush the way that an eraser would. So it's not that there's some sort of setting that is making a very subtle or even completely non-existent effort visually. Any idea what's going on?

G: Yeah — that symptom set points to one of three very specific possibilities in GIMP, and I can narrow it down sharply based on what you described.

Let’s go step by step.

1. The core reason the eraser “paints” instead of erasing

When a layer has no alpha channel, GIMP has nowhere to put transparency — so “erasing” just paints with the background color.
That explains your first behavior perfectly.

Adding an alpha channel was the right move.
But now the eraser does nothing — that’s the part that indicates one of the following:

2. Most likely cause: Layer Lock

Check the lock icons in the Layers panel:

Lock pixels (the paintbrush icon) or
Lock alpha (the checkerboard icon)

If either of those are active, you won’t be able to erase.
Alpha lock means transparency can’t change — so the eraser literally can’t make pixels transparent.
Pixel lock means you can’t paint or erase at all.

Even a single accidental click on those icons will cause exactly what you describe.

3. Second possibility: Active Selection

If there’s an active selection — even one off-screen — the eraser will only work inside that selection, and sometimes nothing happens visually if you’re outside it.
Try:

Select → None
or
Ctrl+Shift+A

That often restores normal tool behavior immediately.

4. Third possibility: Layer Mask Instead of Layer

Sometimes, when pasting or working in Multiply mode, GIMP auto-focuses on the layer mask instead of the layer itself (white border around the mask thumbnail instead of the image thumbnail in the Layers panel).
If that happens, the eraser won’t touch the visible image — it’ll only modify the mask (which might appear to do nothing if the mask is pure white or fully transparent).

Just click the image thumbnail (not the mask) in the Layers panel to ensure you’re editing the layer content itself.

5. Minor but worth checking

Tool options: make sure Eraser mode is Normal, not something like “Color Erase” or “Anti Erase.”
Opacity and Brush dynamics: check that the Opacity is indeed 100% and Flow or Pressure sensitivity aren’t throttling it.
Image Mode: ensure you’re in RGB, not Indexed (Indexed can silently block transparent edits).

🔧 Diagnostic Shortcut

Run through these in this exact order:

Ctrl+Shift+A (clear any selection).
Click the image thumbnail in Layers panel.
Ensure no lock icons are active.
Look at the bottom of the window — confirm “RGB” mode.
Try the eraser again.

If it suddenly starts working, you’ll know which one it was by elimination.

If you want, you can tell me the tiny indicators you’re seeing in the Layers dock (the icons around the layer thumbnail), and I can pinpoint which lock or mode you accidentally fell into. GIMP’s icon feedback is subtle and a bit maddening that way.