OnMouseDown event

Raised when the user presses a mouse button over the view. The Sender parameter refers to the TPdfView instance that raised the event, so a single handler can serve multiple views by inspecting Sender

Typical uses include starting a custom selection box, beginning panning, or capturing coordinates for a "go to this annotation" feature. Because the handler runs on the VCL main thread, you can update form controls and call other VCL APIs directly

Handlers must return quickly — PDFium rendering and form filling occur on the same thread, and any delay here postpones the next page repaint

• Set TabStop to True if you want this control to participate in tab-key focus traversal so keyboard events are delivered
• When you assign this event at design time, the IDE generates a stub procedure on the form; you can also assign it at run time with PdfView1.OnMouseDown := MyHandler
• Combine it with the matching Set Cursor / Set Caret calls on TPdfView to keep visual feedback in sync