A larger GIMP canvas does not necessarily make every layer larger. If painting works over the original photograph but stops at a rectangle inside the new border, inspect the active layer boundary. The layer may still have its old dimensions. Enlarge the required layer bounds or use a separate border layer rather than stretching the photograph to make room.
Identify three different rectangles
The image canvas defines the final visible page. A layer has its own extent, which may be smaller or larger. A selection controls which permitted pixels receive an operation. These are independent constraints. Clearing a selection cannot create storage beyond a small layer, and increasing the canvas does not by itself explain which layer will receive a brushstroke.
GIMP documents a dashed layer boundary as a useful clue when a tool is acting outside a layer. In GIMP 3, some paint tools also offer Expand Layers. Check that option deliberately if you intend automatic growth; do not assume that every tool or every saved setup has it enabled.
A border example without scaling the subject
Consider a square photograph placed in a larger rectangular canvas for a caption margin. The photo should keep its original proportions and pixel dimensions. A separate solid-color layer underneath can cover the canvas while the photo stays untouched. This is often clearer than forcing the image layer to serve simultaneously as photograph, border, and caption background.
If paint truly belongs on the image layer, GIMP's Layers to Image Size or Layer Boundary Size commands can extend its extent. These commands concern the layer's bounds. They should not be confused with Scale Layer, which resamples and changes the size of image content. Read the command and the preview before accepting it.
Verify the limit with a small test
This proposed test has not been executed here. Save a duplicate, show the layer boundary, and make a short stroke across the old photo edge into the new canvas. Note the exact stopping point, then undo. Remove an unintended selection and repeat without changing layer dimensions. If the stopping rectangle remains, selection removal alone did not resolve the layer extent.
Next extend only the intended layer boundary, or activate a full-canvas background layer, and repeat the test. Record canvas dimensions, layer dimensions, and the active layer name. Keeping those three observations is more useful than a screenshot of a failed brush cursor without context.
For an Imagild reference, reopen the exported copy and verify that the subject was neither stretched nor accidentally cropped, the border has the intended color or transparency, and all temporary strokes are gone. Enlarging a border creates layout space; it does not reveal missing photographed surroundings. Use generative expansion only when a new interpretation of those surroundings is actually part of the brief.