Limits and silent failures

What this page is for

The format is fully decoded, so "can I write it" is answered by the reference. This page answers the other two questions: what can Magma not express at all, and what will it accept and then quietly not do. Numbers come from the wire format; frequencies from the 50-package vanilla corpus.

Hard limits of the format

Limit	Value	Consequence
Type-table entries	254 (a single count byte); shipped files use 167	Not reachable in practice — every constructible class is already declared
Geometry	u16 everywhere: LEFT/RIGHT/TOP/BOTTOM, POSITION, ORIGIN, STATICBOX, HOTSPOT	Coordinates are −32,768…32,767 in whole pixels. No sub-pixel positioning; a negative is written 65536 + v
Keyframe IDX	read as u32, stored truncated to u16	Frame numbers above 65,535 silently alias
MASKMODE	u32 on the wire, low 3 bits kept	Only 4 legal values
BLENDINGMODE	u32 on the wire, low byte kept	27 defined modes
EditBox maxLength	u32 on the wire, low 16 bits kept
Colour	32-bit ARGB, 8 bits per channel	No HDR, no per-channel blend factors beyond the blend mode
Slider range	RANGEMIN/RANGEMAX u32; 24 of 33 use 0–10	Integer only

Closed sets you cannot extend

Each of these is an ancestor walk against a hardcoded list in the engine. A type outside the set produces a null object the caller dereferences without a null check, so it is a crash, not a degradation — and the JackAll encoder rejects it before you get there.

• 5 area types: Area, Page, Button, Cursor, CheckBox.
• 14 widget types. There is no way to define a fifteenth. Every custom-looking control in the game — the value spinner, the tab strip, the weapon-shop grid — is a composition of these.
• 9 action-executer types, and the event array each one carries is fixed-length.
• The action registry. ACTIONNAME is CRC32(name) resolved by ActionServer::MakeAction against names the game binary registers at startup — 6 from magma::ActionServer::RegisterStandardActions and 81 from CMagmaActionDispatcher::RegisterCustomActions, all hardcoded C++ string literals. You cannot define a new action from data, and an unregistered name gives you nothing to hang behaviour on. The full registry is on the interop page and the 68 names shipped packages actually use are catalogued in the reference; four of them (Stop, Continue, GotoFrameIndex, GotoKeyframe) are general-purpose timeline control, and everything else calls into compiled code for one specific screen.
• The keyframe state class, decided by the widget class alone. An Image cannot have a ScaleState; to scale a picture, instance an area that contains it.

What Magma has no concept of

• Layout. There is no anchoring, no percentage sizing, no flow, no auto-layout of any kind. Every coordinate is absolute within its area. The only relative mechanism is that an instanced area's contents move with the instance's POSITION, and ScaleState's SCALEX/SCALEY.
• Resolution independence. PAGESIZE is a fixed design canvas per aspect variant, and the two shipped variants have different geometry, not a scale factor. See aspect variants.
• Scripting or expressions. No conditionals, no arithmetic, no data binding. A value that changes at runtime is written by native code into a named widget; the package's job is to provide the widget and the name.
• Text measurement or rich text. WRAPPING, CLIPPING, ELLIPSIS and AUTOSIZED are the whole layout vocabulary for a string. One font family and one HEIGHT per Text element — no runs, no inline colour, no markup. This is why templates carry Wwwwwwwwwww placeholders: sizing is done by eye at author time.
• New fonts without a font package. Font families resolve out of \fonts.mgb; the corpus uses exactly two. Embedding a font blob is possible (<FONTSUBST> carries one), but no shipped package does it, so that path is untested.
• Vector drawing. RectShape (optionally outlined, four corner colours) is the only untextured primitive. Everything else is a textured quad.

Decoded but never exercised

These are writable and structurally understood, but zero shipped packages use them — treat them as unexplored rather than supported:

Feature	Corpus uses
Window (the 9-patch border widget)	0 elements
AutonomousAreaInstance	0
RadioButtonInstance	0
<STRINGTABLE> / useStringTable="true" on Text	0 packages
MASKMODE="USEMASK_INVERTED"	0
ISDUPLICATABLE="false"	0 of 5,422 elements
<DEFAULTFOCUSES> entries on a PageInstance	0
INTERPOLATION="CircleDecel"	0 keyframes
INPUTFILTER or CONTROLLER other than 255	0
21 of 27 blending modes	0

Failures that are silent

The engine's own XML loader treats every element as optional and degrades quietly. The binary loader has no validation at all past the header. So most authoring mistakes produce a screen that draws the wrong thing, not an error:

Mistake	What happens
Page name not in the GenericObjectTable	CUIPageBase::Init finds nothing, binds nothing. Package loads; page is blank. An empty name short-circuits Init entirely — harmlessly
p_menu_nav / l_menu_nav_list / a_title_bar / t_page_title missing or misnamed	FetchMagmaElements misses, AddButton returns −1, rows silently never appear
A SETTING_* UserData key the native side asks for is absent	GetUserDataElement misses; that row has no value control
Material PACKAGE written as common instead of \common.mgb	Material does not resolve; the image draws as an untextured white quad. A full-screen one washes out the whole page
Package identified by a path other than the one its texture paths resolve against	Same white-quad failure, for every image at once — this is a real FCSE bug, fixed by naming the package UI\fcse.mgb
AreaLink PACKAGE written as a path instead of a bare-name hash	Instance resolves to nothing; the sub-tree is missing
ISUSINGDUPLICATEDAREA="false" on a list row template or a repeated button	All copies share one playhead and animate together
More list items than BUTTONCOUNT	Fine for the list (it scrolls), but absolutely positioned siblings do not scroll with it
Element with zero keyframes	No geometry; nothing drawn
Wrong RectState field order (l/t/r/b instead of l/r/t/b)	A plausible rectangle in the wrong place
Colour read as RGBA instead of ARGB	00FFFFFF looks like opaque cyan instead of transparent white
A name that collides under CRC32 within a scope	Undefined which object a reference finds

jackall mgb verify <file.mgb\|.xml> catches the rows above that a single file can answer for: every reference to a name the package itself declares, and — with --page <NAME> — whether that page is registered in the GenericObjectTable under the name the native side will ask for. References into another package are the engine's to resolve against whatever else is loaded, so it says nothing about those.

Failures that crash

Short list, and all of them are prevented by using the encoder rather than splicing bytes:

• A type slot resolving to a class outside MakeArea / MakeElement / MakeState / MakeActionExecuter — the result is dereferenced with no guard.
• A truncated or mis-sized record. The format has no lengths and no sentinels, so one wrong field width shifts every subsequent read; the file will not usually fail, it will read garbage as structure.
• Growing the type table by splicing (every body offset after it shifts). Reserialise the whole package instead — which is what jackall mgb encode does.

The practical envelope

Within all of that, what you can build is wider than the vanilla UI suggests: arbitrary composition depth through instancing, per-element keyframe animation with six easing curves, gradient-tinted quads, stencil masking, 9-patch borders (untested), Bink video playback via the BINKFILENAME area property, and full timeline state machines driven by Stop/Continue/ GotoFrameIndex with no native code at all.

What you cannot do is invent behaviour. Every interaction ultimately lands on an action name the game already registers, or on a native page class that looks your widgets up by name. Design the screen in Magma; put the logic in a plugin.