User Interface

The workspace is one browser tab: a top bar with the open files, a toolbar, a panel column on the left, and the viewport. The same layout serves parts and assemblies; the toolbar and the panels change with the kind of file that is open.

The part workspace

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1. 1
   File tabs

   Every open file is a tab. The current model tab owns the viewport; drag to reorder, right-click to rename.
2. 2
   + (Open a file)

   Opens a box listing every file in the workspace. Pick one to open it as a tab, or type a name that does not exist to create it.
3. 3
   Import

   Imports a STEP file into imports/ and appends a load() statement.
4. 4
   Export

   Lists the scene’s solids; pick one for the STEP / STL / PNG dialog. In an assembly the list leads with Whole assembly.
5. 5
   Toolbar

   Sketch, the create features (Extrude … Hole), the pick tools (Fillet, Chamfer, Shell, Offset), datums and transforms, Boolean, and the structure tools Part and Connector. Undo / Redo sit at the left end.
6. 6
   Feature tree

   Shows or hides the panel column.
7. 7
   Code editor

   Docks the source editor on the left (Ctrl+B). The editor takes width from the viewport rather than covering it.
8. 8
   History

   One row per feature, in file order. See below for what clicks do.
9. 9
   History menu

   Recompute scene (clears the cache and rebuilds everything) and Show execution time.
10. 10
    Shapes

    One row per solid. Click to select it; the row menu hides it, sets transparency, or exports it.
11. 11
    Parameters

    One control per param() and one row per property(). + adds a parameter or a property, the arrow resets all values to their defaults.
12. 12
    Shape properties

    Volume, surface area, mass and centre of mass of a solid or of a whole part, from the part’s material.
13. 13
    Unit

    The document unit. Readouts and dialogs use it; the menu switches the display to mm / in / same as project.
14. 14
    View cube

    Click a face or an axis dot for that view. Below it: fit to view, and the grid / projection toggles.

With the code editor open, the file sits between the rail and the panels. Edits in the editor re-render on save, and features applied from the dialogs appear in the editor as they are written:

Settings

The gear in the top bar, just before the light/dark switch, opens the global settings. They are yours rather than the project's, and they apply everywhere FluidCAD runs for you: the browser, the VS Code extension and the desktop app. Edits wait for Save; a tab with an unsaved edit shows a dot, and Cancel drops the edits.

Tab	What it holds
Appearance	Dark or light theme. The sun and moon button in the top bar switches it too.
Editor	The code editor's font, picked from the monospace fonts installed on the computer, its size, word wrap, and whether the editor pane opens when FluidCAD starts (off by default).
Sketch	The snap radius and the pick radius, in screen pixels: how close the cursor has to be for the sketcher to snap to a vertex, axis or grid line, and for a line, arc or vertex to highlight under it.
Timeline	Which rows the History panel lists under a sketch: all of its children or only the editable features (the ones with an edit dialog, such as offset, projection and intersect), whether its N constraints group shows (on by default), and whether its N regions group shows (off by default).
Units	The unit new projects are created in. Existing projects keep theirs; change those from the unit chip at the bottom of the viewport.
Materials	Your own materials — name, id, density and unit — kept on this machine and offered beside the built-ins wherever a material is picked. See Your own materials.
Advanced	Reset every setting to its default, after a confirmation. This one acts at once rather than on Save.

Project settings, such as the engine pin and the project unit in fluidcad.json, are not here; see Project configuration.

The History panel

The History panel is the feature timeline. Every statement in the file is a row: sketches fold their geometry, an N constraints group and an N regions group (the region declarations for extrude-family picks; the group is off by default), parts fold their features plus N connectors and N exposed groups. The Timeline tab of the settings picks which of a sketch's rows are listed. The glyph on the right of a row says whether the feature was served from cache (check) or rebuilt (refresh arrows). A sketch, plane or axis a feature has used is dimmed — the feature took it off the screen — and carries an eye on hover: click it to draw the object again, and again to hide it. The eye is view state only; nothing in the file changes.

• Single click a row to roll the model back to that feature. The viewport shows the model as it was right after that row, and the toolbar shrinks to what makes sense there. Click the last row, or edit the file, to come back.

• Double-click a row to edit the feature: a breakpoint is placed after it, so the model rolls back to that point, and the feature's dialog opens with its current values. Apply writes the change into the existing statement.

• Right-click a row for its menu (a part row also carries a ⋮ button at its right edge that opens the same menu):

  

  Rename edits the feature's .name('…') in place. Edit feature is the double-click action; on a sketch row it reads Edit sketch, takes a .close() chain off the statement and reopens the sketch at a breakpoint. Breakpoint here puts a breakpoint() after the row without opening a dialog, so features you add next land after that row instead of at the end of the file. Remove deletes the statement; if later features depend on it, a dialog lists them and deletes the closure on confirm. A part row also offers Set material…, described under Shape properties.

• The ⋮ menu on the panel header:

  

  Recompute scene clears the geometry cache and rebuilds every feature from scratch. Use it when a cached result looks stale. Show execution time annotates each row with its build time, and marks whether it was rebuilt or came from the cache.

Breakpoints

A breakpoint() statement stops evaluation at that line. The viewport shows the model up to that point, and anything the toolbar writes is inserted before the breakpoint rather than at the end of the file. Remove the statement to resume the full model. Besides the History menu, VS Code inserts one from the gutter and Neovim with :FluidCadToggleBreakpoint.

After a complete evaluation, History keeps later features visible when a breakpoint stops the next build. These dimmed rows carry a pause icon: their names and structure come from the last complete evaluation, and they have not been evaluated in the current build. You can expand them and follow their source, but geometry and editing actions become available when they are evaluated again.

Reloading the page preserves this history while the server is running. Opening a file already paused without a previous complete evaluation, or restarting the server, shows only evaluated features until the file completes a build. A source edit that removes a statement or makes its identity ambiguous drops its remembered row.

Shapes and shape properties

The Shapes panel lists the solids the model produced, grouped by type. Clicking a row selects the solid in the viewport; the row menu toggles visibility, sets transparency, and opens the export dialog for that solid.

The scale icon at the bottom right opens Shape properties. The Part | Solid tabs at the top choose what the numbers describe, and Calculate fills Volume, Surface Area, Mass and Center of Mass in the chosen length and mass units. A density is never typed: mass comes from a material, and Density under it is read-only.

• Solid (the default) measures one solid: click a face or edge in the viewport, or a row in the Shapes panel. A solid inside a part that declares a material shows Material and Density read-only — the part owns them.

  

  A solid outside any part has a Material dropdown instead, grouped Built-in and Custom — the project's fluidcad.json entries and your own from Settings → Materials. The pick is for this calculation only; nothing is written to the file. Manage materials… under it opens Settings → Materials.

  

• Part measures a whole part: click any of its solids in the viewport — every solid of the part highlights — or click the part's row in the History panel; in an assembly, click an instance. Calculate sums the part's final solids, the ones the Shapes panel lists under it. The material is read-only here; it is set from the History panel.

  

Setting a part's material

Click the ⋮ button at the right edge of the part's row in the History panel (or right-click the row) and choose Set material….

The Set material dialog opens. None is on top and removes the material; Built-in lists the built-in materials and Custom the custom ones — the project's own, the materials map of fluidcad.json, tagged in project, followed by yours from Settings → Materials. The current material is checked and sits first in its group; every row shows its density, and the box at the top filters by name or id (the current material stays listed). While the list is on its way the dialog says Loading materials…, and if it cannot be fetched it says so with a Retry. Click a row and Apply (or double-click it, or press Enter): .material('<id>') is written on the part() statement — see Part — and the panel picks it up on the next render. Manage materials… at the bottom left opens Settings → Materials.

A material of yours that the project has not used yet is copied into the materials map of fluidcad.json the moment it is applied, so the file names an id the project resolves on every machine.

An id that neither list knows — a typo in the file, or a project entry that was removed — is a warning, not an error. The geometry builds, the row carries a warning triangle (hover it for the message), and the panel reads Unknown material: <id> with no mass:

Your own materials

The Materials tab of the settings — the gear in the top bar, or Manage materials… in the Set material dialog and under the dropdown in Solid mode — holds your own materials. They are kept on this machine, in the preferences file, not in a project.

The list shows each entry's name, id, density and unit; the pencil loads it into the form, the bin removes it. Add material takes a Name, a Density with its Unit (g/cm³, kg/m³, g/mm³ or lbs/in³) and an Id, filled from the name until you type your own; Add puts the entry in the list. Like every settings tab this is a draft: Save persists it, Cancel drops it. Built-ins are not listed and cannot be edited, but an entry may reuse a built-in's id: set on a part, it overrides that built-in in the project it is copied into.

A project carries exactly the custom materials it uses: applying one of these to a part copies its entry into materials in that project's fluidcad.json, which is what a teammate's render and the viewer resolve against. Removing an entry here touches no project, and nothing prunes a project's map. Editing fluidcad.json by hand works as before; Project configuration has the schema.

Parameters

The Parameters panel shows one control per param() in the file: a number field, slider, text box, dropdown, checkbox or color swatch, according to the declared type. Changing a control writes the new value into the source and re-renders. Below the controls, a Properties list shows each property() of the part by its label, with the value the last render computed. + opens a menu: New parameter or New property, each declared through a dialog; the pencil on a row edits or deletes its declaration, and the arrow resets every parameter to its default. Renaming a declaration renames the variable it binds and every read of it, in other files too; deleting one puts its value in place of its reads, or refuses when the value would not mean the same where it is read. See Param and Property.

Edges in the viewport

The viewport draws the edges of the model as lines. Two kinds of B-rep edge are not model edges and are treated accordingly:

• Seams — the line where a cylinder, cone or sphere's surface closes on itself — are never drawn. They are not selectable either: select(edge()), the feature accessors such as sideEdges(), and edgeCount() never return or count one, so a boss looks and behaves the same whichever way its sketch plane happened to orient the seam.
• Tangent edges — where two faces meet smoothly, as at a fillet's boundary or where a flat flank runs into a cylinder — are drawn like every other edge and remain selectable. Setting dimTangentEdges to true in the preferences file draws them dimmed, pulled toward the face colour, so only real creases read as lines.

Measuring

There is no measure tool to arm. Click a face, edge or vertex in the viewport and a status bar appears with its type and its measurement (area for a face, length for an edge). Ctrl+click or Shift+click adds entities: two faces give the minimum and maximum distance between them, two parallel faces the parallel distance, two cylinders the axis distance, two edges an angle.

Click Show all measurements on the status bar for the full panel, with one card per selected entity and the pairwise results.

Measuring works the same in an assembly: click any part's face or edge, and the live pose of the instance is taken into account.

The assembly workspace

Opening a .assembly.js file swaps the toolbar for the assembly tools and the panels for the assembly rail.

• Toolbar: Insert (a dialog listing every part and sub-assembly in the workspace), the joints Fastened, Revolute, Slider, Cylindrical, Planar, Tangent, then Replicate and Connector (a free frame in assembly space).
• Parts: one row per inserted instance, its name over the part it was inserted from; the eye hides it, the menu offers Show in source, Toggle grounded, Rename (which renames the instance's variable too) and Delete. Sub-assemblies group their members under a header.
• Connectors: the assembly-level connectors declared in this file. A connector with copies heads a family: the row shows its size and a chevron that folds out one instance(k) row per copy. The menu offers Show in source, Copy… (Edit copy… once copied) and Delete; on a copy, Show in source, Edit copy… and Remove copies.
• Joints: one row per mate(), with the two parts and the limits. Click a row to highlight both connectors; the menu offers Animate… (revolute and slider), Edit mate…, Suppress, Delete and Show in source.
• Instances are moved by dragging them, or with the gizmo's arrows and rings; the new pose is written back to the insert() statement. Dragging a jointed part moves it within the joint's freedom.

See Assembly for the joints and the Getting started chain for the full flow.

Undo and redo

Every toolbar operation is one edit to the source file, so Undo and Redo step the editor's own history: the first two buttons on the toolbar, Ctrl+Z and Ctrl+Shift+Z (Ctrl+Y also redoes; on macOS use ⌘). The keys work in both workbenches with the viewport focused; inside the code editor or a text field they keep their native meaning. The group appears once an editor is attached — the in-page editor, or the VS Code / Neovim extension.

Editors

The in-page editor is enough for most work: it completes against the engine's own type declarations and shows diagnostics inline. The VS Code and Neovim extensions drive the same server from your editor; see Editor setup.