Flash Player Oddities

This page is a list of weird or buggy behaviors in the official Flash Player. Many of these issues are not well understood and require investigation. This also includes behavior that varies between different version of the Flash Player. Spooky! 👻

AVM1

DefineFunction2 skips preloading _parent register

If a DefineFunction2 function on the root timeline tries to preload _parent in a register, the player ends up skipping this preload because _parent is undefined. _global will mistakenly get preloaded in this register instead if the function also uses it. See the define_function2_preload_order test for an example.

NaN == NaN is true

NaN == NaN returns true in AVM1 depending on the version of the Flash Player. Flash Player 6 and below(?) properly returns false. Higher versions return true. This is not affected by the SWF version.

NaN.toString(base) returns garbage for non-decimal bases

NaN.toString(16) returns -(0000000 for example. There are different garbage values for every non-decimal base. The int-to-string algorithm ends up chewing on the bit pattern for -NaN (0xFFF80000_00000000), getting negative values for the digit and indexing the wrong way from ASCII '0'.

Some simple code that should match the result of Flash Player for NaN: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=95ff82ad6762025e08556ace0e061ae0

This depends on the version of the Flash Player, regardless of SWF version. It seems to occur Flash Player 7? and higher. Flash Player 6 and below prints 0 for all non-decimal bases. Currently Ruffle returns 0 in this case, but we may want to make this a setting in the future.

o is null

trace(o) will print null instead of the expected undefined.

Trace (opcode) != trace (_global function)

The opcode Trace will output to flashlog.txt, but _global.trace will not (instead, only showing up in the debugger console)

AVM2

stage.transform

stage.x, stage.scaleX throw an error when accessed. However, stage.transform is still accessible. Changing stage.transform.matrix does transform vector art on the stage, but other objects such as TextFields are not affected!

Addtionally, stage.transform.concatenatedMatrix doesn't return an identity matrix by default. It returns a scale matrix depending on the stage quality level, e.g. High stage quality is 4x supersampling and results in a matrix that scales by 4x.

stage.tabChildren

stage.tabChildren always returns true when read, but when set, it actually sets the value of tabChildren of the root movie clip. The weird thing is that the tabChildren of the root is set even when the root is removed (e.g. by stage.removeChild(root)).