Preserve the first mouse movement of a drag

When the left button is pressed and the pointer then moves, the event
that made the MAYBE_DRAGGING -> DRAGGING transition fell through the
switch without being assigned a key or modifier, so the first cell of
every drag arrived at handlers as a MouseRelease event without the
motion modifier and was effectively lost. Give it the same
MouseLeft/ModMotion identity as all subsequent drag events.

Held-button motion events that stay within the pressed cell carry no
information at all; swallow them instead of letting them through as
further release-shaped events (which used to clobber the double-click
state when the pointer jittered within a cell between two clicks).

pkg/gocui/tcell_driver.go

@@ -366,7 +366,9 @@ func gocuiEventFromTcellEvent(tev tcell.Event) GocuiEvent {
 
 		// process button events (not wheel events)
 		button &= tcell.ButtonMask(0xff)
+		newButtonPress := false
 		if button != tcell.ButtonNone && lastMouseKey == tcell.ButtonNone {
+			newButtonPress = true
 			lastMouseKey = button
 			lastMouseMod = tev.Modifiers()
 			switch button {
@@ -410,9 +412,23 @@ func gocuiEventFromTcellEvent(tev tcell.Event) GocuiEvent {
 				}
 			// if we haven't released the left mouse button and we've moved the cursor then we're dragging
 			case MAYBE_DRAGGING:
-				if x != lastX || y != lastY {
-					dragState = DRAGGING
+				if x == lastX && y == lastY {
+					// Deliver the button press itself, but swallow held-button
+					// motion events within the same cell: they carry no new
+					// information, and if they fell through they would be
+					// delivered with the default MouseRelease key.
+					if !newButtonPress {
+						return GocuiEvent{Type: eventNone}
+					}
+					break
 				}
+				// The first movement is already part of the drag; give it the
+				// same key and modifier as the DRAGGING events below so it
+				// reaches drag bindings instead of being delivered with the
+				// default MouseRelease key.
+				dragState = DRAGGING
+				mouseMod = ModMotion
+				mouseKey = MouseLeft
 			case DRAGGING:
 				mouseMod = ModMotion
 				mouseKey = MouseLeft

pkg/gocui/tcell_driver_test.go

@@ -0,0 +1,33 @@
+package gocui
+
+import (
+	"testing"
+
+	"github.com/gdamore/tcell/v3"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFirstMouseMovementAfterPressIsDragEvent(t *testing.T) {
+	t.Cleanup(resetMouseState)
+	resetMouseState()
+
+	pressEvent := gocuiEventFromTcellEvent(tcell.NewEventMouse(1, 2, tcell.ButtonPrimary, tcell.ModNone))
+	unchangedHeldEvent := gocuiEventFromTcellEvent(tcell.NewEventMouse(1, 2, tcell.ButtonPrimary, tcell.ModNone))
+	dragEvent := gocuiEventFromTcellEvent(tcell.NewEventMouse(1, 3, tcell.ButtonPrimary, tcell.ModNone))
+
+	assert.Equal(t, eventMouse, pressEvent.Type)
+	assert.Equal(t, MouseLeft, pressEvent.Key.KeyName())
+	assert.Equal(t, ModNone, pressEvent.Key.Mod())
+	assert.Equal(t, eventNone, unchangedHeldEvent.Type)
+	assert.Equal(t, eventMouse, dragEvent.Type)
+	assert.Equal(t, MouseLeft, dragEvent.Key.KeyName())
+	assert.Equal(t, ModMotion, dragEvent.Key.Mod())
+}
+
+func resetMouseState() {
+	lastMouseKey = tcell.ButtonNone
+	lastMouseMod = tcell.ModNone
+	dragState = NOT_DRAGGING
+	lastX = 0
+	lastY = 0
+}