Windows debugger: Make the left edge of the register category tabs clickable

The 16 pixel left margin of the register rows was excluded from every click,
including on the tab row, which has no margin, so most of "GPR" didn't
respond.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Windows/Debugger/CtrlRegisterList.cpp

@@ -559,8 +559,9 @@ void CtrlRegisterList::editRegisterValue()
 void CtrlRegisterList::onMouseDown(WPARAM wParam, LPARAM lParam, int button)
 {
 	int x = (s16)LOWORD(lParam); 
-	int y = (s16)HIWORD(lParam); 
-	if (x>16)
+	int y = (s16)HIWORD(lParam);
+	// The left margin only belongs to the register rows, the category tabs span the whole width.
+	if (x > 16 || y <= rowHeight)
 	{
 		oldSelection=selection;
 
@@ -655,6 +656,11 @@ void CtrlRegisterList::onMouseUp(WPARAM wParam, LPARAM lParam, int button)
 		ReleaseCapture();
 		redraw();
 	}
+	else
+	{
+		// A click on the category tabs captures the mouse too, at any x.
+		ReleaseCapture();
+	}
 }
 
 void CtrlRegisterList::onMouseMove(WPARAM wParam, LPARAM lParam, int button)