Drawings
A tikzpicture opens in the figure editor. Draw on a canvas and Texpile writes the TikZ into your .tex file.
The picture in the paper
In the visual editor a tikzpicture shows as a card with its last drawing. A picture that was never drawn shows a placeholder with a summary of what it holds.
Hover the card and click Edit figure (the pencil) to open the picture in a figure tab next to the paper.
The figure tab
Draw with the tools: boxes, circles, lines, arrows, text and more. Move and stretch shapes on the canvas. Style the selected shape in the inspector on the right. Add a ready-made element from the library.
Every canvas edit is written into the TikZ source in the .tex file, in place. The rest of the file stays as it was.
Undo and redo
The file has one history. Undo and redo in the figure tab go through it.
Undoing in a figure tab a change that was made outside the figure asks first: "Undo Change Outside the Figure?".
The picture in the source
With the cursor in a picture, a chip Edit figure sits on its first line. Click it to open the figure tab.
The source editor marks an open figure. The picture's lines are tinted and a chip "Figure N is open" sits on it. Click the chip to show the figure tab. The statement you are editing in the figure is highlighted.
LuaLaTeX
The drawing comes from your own LuaLaTeX. Texpile starts it when a figure tab opens and keeps it running while a figure tab is open. It stops a minute after the last figure tab closes.
If LuaLaTeX is missing, the figure tab says "Figures need LuaLaTeX." and Texpile offers its usual help to install a TeX distribution. See Install a TeX distribution.
Pictures in other files
A picture in a file that the main file loads with \input is drawn with the main file's preamble. A new shape that needs a TikZ library adds \usetikzlibrary to the main file's preamble.
Unfinished and deleted pictures
When the picture's source is unfinished, for example a missing \end{tikzpicture}, the figure tab asks you to finish it and offers Show in source.
When the picture is deleted from the source, the tab says "This figure was deleted."
.tikz files
A .tikz file opens in the figure editor too. The Visual and Source switch changes between the canvas and the code.
Saved drawings
Texpile keeps the drawings in .texpile/figures. The paper shows them while LuaLaTeX is stopped. That folder stays out of Git.