void __fastcall TForm1::PaintBoxMouseDown(TObject *Sender,
TMouseButton Button, TShiftState Shift, int X, int Y)
{
int shft = Shift.ToInt();
if (shft == 34) { /* doSomething() */ } // Alt / Wheel Click
}

Button gives you the mouseClick that fired the event: 0 = Left; 1 = Right, 2 = Wheel

Shift gives you the status of all three Buttons (Left, Right, Wheel and double clicks) plus all three Shift keys (Cntl; Alt; Shift). A double-click adds bit 7 (decimal 64) to the sum of the different independent keys clicked.

X and Y give you the coordinates where the MouseDown occurred.

Shift monitors the Cntl, Alt and Shift keys as well as the Left, Right, and Wheel Mouse Buttons for click and double-click.
It provides data on the states of all seven variables: "0" means it is not down. "1" means that it is down.
That data is stored as 7 bits according the the table below.
Please note that Shift can represent every combination of those 7 states, a total of 128 in all.
Shift provides us with the 7-bit binary code. Shift.ToInt() will translate Shift into its decimal equivalent.
"If" statements can distinguish these combinations, for example:

int shft = Shift.ToInt();
if (shft == 20) { /* doSomething() */ } // Cntl / Right Click

For most applications, we should confine ourselves to 1 button press and 1 key press; these combinations are colored yellow.
This gives us 12 relatively simple combinations to work with.

Shift.ToInt() Left Right Wheel nokey 8 16 32 shift 9 17 33 alt 10 18 34 shift/alt 11 19 35 ctrl 12 20 36 shift/ctrl 13 21 37 ctrl/alt 14 22 38 ctrl/alt/shift 15 23 39

The 64 possible combinations in the long chart at the bottom of this page have been colored according to the number of Button and Key presses. Combination Button presses can also be captured. Not Available for MouseDown   Recommended: 1 Button / 1 Key   1 Button / 2 keys   1 Button / 3 Keys   2 Buttons / any Keys   3 Buttons / any Keys   Double-Clicks Combinations

 

Shift.ToInt()
Decimal	Double	Wheel	Right	Left	Cntl	Alt	Shift
0		0	0	0	0	0	0
1		0	0	0	0	0	1
2		0	0	0	0	1	0
3		0	0	0	0	1	1
4		0	0	0	1	0	0
5		0	0	0	1	0	1
6		0	0	0	1	1	0
7		0	0	0	1	1	1
8 L		0	0	1	0	0	0
9		0	0	1	0	0	1
10		0	0	1	0	1	0
11		0	0	1	0	1	1
12		0	0	1	1	0	0
13		0	0	1	1	0	1
14		0	0	1	1	1	0
15		0	0	1	1	1	1
16 R		0	1	0	0	0	0
17		0	1	0	0	0	1
18		0	1	0	0	1	0
19		0	1	0	0	1	1
20		0	1	0	1	0	0
21		0	1	0	1	0	1
22		0	1	0	1	1	0
23		0	1	0	1	1	1
24		0	1	1	0	0	0
25		0	1	1	0	0	1
26		0	1	1	0	1	0
27		0	1	1	0	1	1
28		0	1	1	1	0	0
29		0	1	1	1	0	1
30		0	1	1	1	1	0
31		0	1	1	1	1	1
32 W		1	0	0	0	0	0
33		1	0	0	0	0	1
34		1	0	0	0	1	0
35		1	0	0	0	1	1
36		1	0	0	1	0	0
37		1	0	0	1	0	1
38		1	0	0	1	1	0
39		1	0	0	1	1	1
40		1	0	1	0	0	0
41		1	0	1	0	0	1
42		1	0	1	0	1	0
43		1	0	1	0	1	1
44		1	0	1	1	0	0
45		1	0	1	1	0	1
46		1	0	1	1	1	0
47		1	0	1	1	1	1
48		1	1	0	0	0	0
49		1	1	0	0	0	1
50		1	1	0	0	1	0
51		1	1	0	0	1	1
52		1	1	0	1	0	0
53		1	1	0	1	0	1
54		1	1	0	1	1	0
55		1	1	0	1	1	1
56		1	1	1	0	0	0
57		1	1	1	0	0	1
58		1	1	1	0	1	0
59		1	1	1	0	1	1
60		1	1	1	1	0	0
61		1	1	1	1	0	1
62		1	1	1	1	1	0
63		1	1	1	1	1	1
64	1	0	0	0	0	0	0
65	1	0	0	0	0	0	1
66	1	0	0	0	0	1	0
67	1	0	0	0	0	1	1
68	1	0	0	0	1	0	0
69	1	0	0	0	1	0	1
70	1	0	0	0	1	1	0
71	1	0	0	0	1	1	1
72 LL	1	0	0	1	0	0	0
73
74
75
76
77
78
79
80 RR
81
82
83
84
85
86
87
88 LRR RLL
89
90
91
92
93
94
95
96 WW
97
98
99
100
101
102
103
104
105 LWW WLL
106
107
108
109
110
111
112 RWW WRR
113
114