New capture-tab capability on the addon-host, and the screenshot add-on
uses it to expose three modes in a sidebar launcher panel:
- Visible viewport: Electron's WebContents.capturePage() on the active tab
- Full scrollable page: temp-resize the tab view to document.scrollHeight,
capturePage, restore
- Region: preload overlays a translucent selection div, tracks mousedown /
move / up, sends the rect back; main takes the visible capture and
crops via nativeImage.crop({x,y,width,height})
Saves land in the user's Downloads folder via session.downloadURL — same
pipeline as any file download, so the download chip picks them up.
Filename: theseus-screenshot-<host>-<ISO date>.png. JPEG option for
smaller files.
A follow-up task (task_b9608dc6) reworks this to open captures in a
full-tab editor with crop / draw / annotate / undo / copy-to-clipboard
instead of the current bare launcher.
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