Inspect the file you will actually share
An editing view and a saved output are separate things.
From source to a new bitmap
MaskHarbor draws the decoded image into a canvas, fills each saved rectangle with opaque black pixels and encodes that canvas as a new PNG. The green selection border belongs to a separate display layer and is not exported. The output is a raster image without MaskHarbor editing layers or an embedded original image.
The browser's canvas PNG encoder creates the output; the app does not copy the source file's EXIF block or original filename into the pixel file. This is not a forensic metadata certification. The PNG can still contain ordinary encoding information, and visible content outside masks remains part of the image.
Review after the last change
Build export preview creates the PNG that the download button will save. Inspect it, then tick the review checkbox. Editing, removing, moving or undoing a mask invalidates that preview, hides it and disables its download. Build a new preview after changes.
Share the exported file
Downloading does not overwrite the original file on your device. The new filename ends in -masked.png. Open the downloaded file in an image viewer for a final check before sharing it. Other files, video frames, attachments and earlier messages are outside this tool's scope. There is no automatic cloud deletion or recipient recall.
Technical reference: MDN canvas toBlob documentation.