There seems to be a mismatch between the location

Question

I'm trying to set the back color of each TextBox that is within the rectangle defined between a MouseDown and MouseUp event, where the MouseUp event takes place to the right of and below the MouseDown event.

I capture the Points this way:

static readonly Color PSEUDO_HIGHLIGHT_COLOR = Color.Gainsboro;
private Point selectionStart; 
private Point selectionEnd; 

. . .

private void flowLayoutPanelGreatGooglyMoogly_MouseDown(object sender, MouseEventArgs e) {
    if (e.Button == MouseButtons.Left) {
        selectionStart = PointToClient(MousePosition);
    }
}

private void flowLayoutPanelGreatGooglyMoogly_MouseUp(object sender, MouseEventArgs e) {
    if (e.Button == MouseButtons.Left) {
        selectionEnd = PointToClient(MousePosition);

        HighlightAllTextBoxValsBetweenPoints();
    }
}

...and here's the code that should set the BackColor of the TextBoxes "beneath" the virtual rectangle described by the Points:

private void HighlightAllTextBoxValsBetweenPoints() {
    TextBox tb;
    foreach (Control ctrl in flowLayoutPanelGreatGooglyMoogly.Controls) {
        if (ctrl is TextBox) {
            tb = (TextBox)ctrl;
            if ((tb.Location.X >= selectionStart.X) &&
                (tb.Location.Y >= selectionStart.Y) &&
                (tb.Location.X <= selectionEnd.X) &&
                (tb.Location.Y >= selectionEnd.Y)) {
                tb.BackColor = PSEUDO_HIGHLIGHT_COLOR;
            }
        }
    }
}

...but, although the code is being executed, none of the TextBoxes are ever seen as being within those constraints.

It's as if I'm dragging over a map of the good ol' USA, but the troll under the bridge is telling me that Kansas is really in Canada.

My logic is probably mixed-up or backwards (failing to compare the X and Y coordinates correctly), or I'm failing to convert the Points from relative to absolute or vice versa.

UPDATE:

The answer was almost perfect (and how was John to know that my FlowLayoutPanel was situated where it was); to get it to work, I just had to add this above his code:

// Have to subtract the Top (Y) value of the Panel 
int DeltaFromFormTopToPanelTop = flowLayoutPanelGreatGooglyMoogly.Location.Y; 
selectionStart.Y = selectionStart.Y - DeltaFromFormTopToPanelTop; 
selectionEnd.Y = selectionEnd.Y - DeltaFromFormTopToPanelTop;

UPDATE to the UPDATE

To prevent the coloring/prompting when a user has only selected one control, I had to add this code, too:

if (e.Button == MouseButtons.Left) {
  selectionEnd = PointToClient(MousePosition);
  if (MouseHasNotMovedFar()) {
    return;
  }
  HighlightAllTextBoxValsBetweenPoints();
  PromptForAndAssignInputValue();
}

private bool MouseHasNotMovedFar() {
// The "X" or horizontal, is TextBoxWidth*2 + LabelWidth*1
// The "Y" or vertical, is TextBoxHeight*2
// If the user has moved the mouse less than these between
// MouseDown and MouseUp, they probably have not dragged to 
// select multiple TextBoxes.
  const int ACCEPTABLE_X_DELTA = 74;
  const int ACCEPTABLE_Y_DELTA = 40;
  return (selectionEnd.X - selectionStart.X) <= ACCEPTABLE_X_DELTA &&
         (selectionEnd.Y - selectionStart.Y) <= ACCEPTABLE_Y_DELTA;
}

Answer 1

There's a few issues.
First - textbox coordinates are relative to the flowLayoutPanel. Therefore your PointToClient must be the same:

selectionStart = flowLayoutPanelGreatGooglyMoogly.PointToClient(MousePosition);

Second - you had a logic bug; the last comparison was reversed. (tb.Location.Y >= selectionEnd.Y)) {

A suggestion to simplify the process and allow dragging in any direction would be to use Rectangles:

private void HighlightAllTextBoxValsBetweenPoints()
{
    var selectionBounds = new Rectangle(
        selectionStart.X, 
        selectionStart.Y, 
        selectionEnd.X - selectionStart.X, 
        selectionEnd.Y - selectionStart.Y);
    foreach (Control ctrl in flowLayoutPanel1.Controls)
    {
        var tb = ctrl as TextBox;
        if (tb == null)
            continue;

        if (tb.Bounds.IntersectsWith(selectionBounds))
            tb.BackColor = PSEUDO_HIGHLIGHT_COLOR;
    }
}