Redesign the logic for scrolling the grid on drags

Update the grid scroll position on mouse up rather than mouse down,
eliminating the need for the logic that made it impossible to do a drag
selection from the top or bottom rows of the grid, and fix some cases
where the grid would scroll in an undesirable direction.

Originally committed to SVN as r6746.

aegisub/src/base_grid.cpp

@@ -373,9 +373,7 @@ void BaseGrid::EndBatch() {
 }
 
 void BaseGrid::MakeCellVisible(int row, int col, bool center) {
-	int w = 0;
+	int h = GetClientSize().GetHeight();
-	int h = 0;
-	GetClientSize(&w,&h);
 
 	// Get min and max visible
 	int minVis = yPos+1;
@@ -422,8 +420,7 @@ void BaseGrid::SelectRow(int row, bool addToSelected, bool select) {
 		AnnounceSelectedSetChanged(Selection(), removed);
 	}
 
-	int w, h;
+	int w = GetClientSize().GetWidth();
-	GetClientSize(&w, &h);
 	RefreshRect(wxRect(0, (row + 1 - yPos) * lineHeight, w, lineHeight), false);
 }
 
@@ -665,10 +662,7 @@ void BaseGrid::OnScroll(wxScrollEvent &event) {
 }
 
 void BaseGrid::OnMouseEvent(wxMouseEvent &event) {
-	int w,h;
+	int h = GetClientSize().GetHeight();
-	GetClientSize(&w,&h);
-
-	// Modifiers
 	bool shift = event.ShiftDown();
 	bool alt = event.AltDown();
 	bool ctrl = event.CmdDown();
@@ -676,55 +670,51 @@ void BaseGrid::OnMouseEvent(wxMouseEvent &event) {
 	// Row that mouse is over
 	bool click = event.LeftDown();
 	bool dclick = event.LeftDClick();
-	int row = event.GetY()/lineHeight + yPos - 1;
+	int row = event.GetY() / lineHeight + yPos - 1;
 	if (holding && !click) {
 		row = mid(0,row,GetRows()-1);
 	}
 	AssDialogue *dlg = GetDialogue(row);
 	if (!dlg) row = 0;
 
-	// Get focus
+	if (event.ButtonDown() && OPT_GET("Subtitle/Grid/Focus Allow")->GetBool())
-	if (event.ButtonDown()) {
-		if (OPT_GET("Subtitle/Grid/Focus Allow")->GetBool()) {
 		SetFocus();
-		}
-	}
 
-	// Click type
+	if (holding) {
-	bool startedHolding = false;
+		if (!event.LeftIsDown()) {
-	if (click && !holding && dlg) {
+			if (dlg)
+				MakeCellVisible(row, 0, false);
+			holding = false;
+			ReleaseMouse();
+		}
+		else {
+			// Only scroll if the mouse has moved to a different row to avoid
+			// scrolling on sloppy clicks
+			if (row != extendRow) {
+				if (row <= yPos)
+					ScrollTo(yPos - 3);
+				// When dragging down we give a 3 row margin to make it easier
+				// to see what's going on, but we don't want to scroll down if
+				// the user clicks on the bottom row and drags up
+				else if (row > yPos + h / lineHeight - (row > extendRow ? 3 : 1))
+					ScrollTo(yPos + 3);
+			}
+		}
+	}
+	else if (click && dlg) {
 		holding = true;
-		startedHolding = true;
 		CaptureMouse();
 	}
-	if (!event.LeftIsDown() && holding) {
-		holding = false;
-		ReleaseMouse();
-	}
 
-	// Scroll to keep visible
-	if (holding) {
-		// Find direction
-		int minVis = yPos+1;
-		int maxVis = yPos+h/lineHeight-3;
-		int delta = 0;
-		if (row < minVis) delta = -1;
-		if (row > maxVis) delta = +1;
-
-		// Scroll
-		if (delta) {
-			ScrollTo(yPos+delta*3);
-			if (startedHolding) {
-				holding = false;
-				ReleaseMouse();
-			}
-		}
-	}
-
-	// Click
 	if ((click || holding || dclick) && dlg) {
 		int old_extend = extendRow;
+
+		// SetActiveLine will scroll the grid if the row is only half-visible,
+		// but we don't want to scroll until the mouse moves or the button is
+		// released, to avoid selecting multiple lines on a click
+		int old_y_pos = yPos;
 		SetActiveLine(dlg);
+		ScrollTo(old_y_pos);
 
 		// Toggle selected
 		if (click && ctrl && !shift && !alt) {
@@ -745,9 +735,8 @@ void BaseGrid::OnMouseEvent(wxMouseEvent &event) {
 		}
 
 		// Change active line only
-		if (click && !shift && !ctrl && alt) {
+		if (click && !shift && !ctrl && alt)
 			return;
-		}
 
 		// Block select
 		if ((click && shift && !alt) || (holding && !ctrl && !alt && !shift)) {
@@ -811,9 +800,8 @@ void BaseGrid::OnContextMenu(wxContextMenuEvent &evt) {
 }
 
 void BaseGrid::ScrollTo(int y) {
-	int w,h;
+	int h = GetClientSize().GetHeight();
-	GetClientSize(&w,&h);
+	int nextY = mid(0, y, GetRows() + 2 - h / lineHeight);
-	int nextY = mid(0,y,GetRows()+2 - h/lineHeight);
 	if (yPos != nextY) {
 		yPos = nextY;
 		if (scrollBar->IsEnabled()) scrollBar->SetThumbPosition(yPos);