I haven't found an official reference for how instructions should behave, so I've taken the disassembly of the original CHIP-8 interpreter for the COSMAC VIP as the reference implementation.
Fortunately, this is rather well documented, and available at http://laurencescotford.co.uk/wp-content/uploads/2013/08/CHIP-8-Interpreter-Disassembly.pdf
The decoding of
Fxxx instructions begin at program address
0105. More precisely, instruction
Fx0A is decoded and executed from address
010A. The execution of this instruction implies to call a ROM routine to read the keyboard:
010A F8 81 FX0A: LDI 0x81 Instruction FX0A -> wait for a key press and
store it in VX
0x81 is the high-order byte of the address
of a routine in the COSMAC VIP ROM that
reads the keyboard
010C BC PHI C Store this in RC.1
010D F8 95 LDI 0x95 0x95 is the low-order byte of the address of
the keyboard routine
010F AC PLO C Put this in RC.0 - RC now contains the full
0110 22 DEC 2 Decrement stack pointer - the ROM routine
uses the stack so we need to ensure the
stack pointer is pointing at the next empty
location before calling it
0111 DC SEP C Call the routine to read the keyboard
On return the value of the key pressed will
be in the accumulator D
0112 12 INC 2 Increment stack pointer
0113 56 STR 6 Store the result in VX
0114 D4 SEP 4 Return to the fetch and decode routine
The behaviour of the keyboard ROM routine is documented by the same guy, Laurence Scotford. This is the flowchart taken from his web (the actual ROM code is also there)
As you can see, the keyboard ROM routine waits for any key to be pressed, and then, the routine waits until it has been released. Then it returns the key code that was pressed
So, according to this, if the Fx0A instruction is executed while a key has been already pressed, the instruction will wait until the key is released and then it will return the key code in register Vx.
Decoding and execution of these instructions begin at address
0199 of the interpreter code. A few instructions later, a direct keyboard reading is performed, isntead of calling a ROM routine.
019A 62 OUT 2 This will take the value in VX and output it
to the keyboard latch. This causes external
flag 3 to be set if that key is currently
held down or reset if not
So these instructions will react accordingly if a key was pressed before the isntruction was executed. Both will read the current state of that key the moment the Exxx instructions are executed. These two instructions don't use the ROM, so they don't interfere with any system variable that would have been modified in response to keys pressed or released.
In other words: if key '1' is pressed and hold, and
V1 stores the key code for '1'...
LD V1,1 6101 ;store key code 1 into V1
...the following sequence of instructions will behave as this:
SKP V1 E19E ;will skip the next instruction.
SKNP V1 E1A1 ;will not skip the next instruction.
LD V2,K F20A ;will wait until the user releases the '1' key,
;then it will store its keycode into