Add Scroll Tab Bar with mouse wheel capacity

Closes #521, closes #522, closes #2071

PowerEditor/src/WinControls/TabBar/TabBar.cpp

@@ -441,6 +441,96 @@ LRESULT TabBarPlus::runProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lPara
 			return TRUE;
 		}
 
+		case WM_MOUSEWHEEL:
+		{
+			// ..............................................................................
+			// MOUSEWHEEL:
+			// will scroll the tab bar area (similar to Firefox's tab scrolling),
+			// it only happens if not in multi-line mode and at least one tab is hidden
+			// ..............................................................................
+			// CTRL + MOUSEWHEEL:
+			// will do previous/next tab WITH scroll wrapping (endless loop)
+			// ..............................................................................
+			// SHIFT + MOUSEWHEEL:
+			// will do previous/next tab WITHOUT scroll wrapping (stops at first/last tab)
+			// ..............................................................................
+			// CTRL + SHIFT + MOUSEWHEEL:
+			// will switch to the first/last tab
+			// ..............................................................................
+
+			if (_isDragging)
+				return TRUE;
+
+			const bool isForward = ((short)HIWORD(wParam)) < 0; // wheel rotation towards the user will be considered as forward direction
+			const LRESULT lastTabIndex = ::SendMessage(_hSelf, TCM_GETITEMCOUNT, 0, 0) - 1;
+
+			if ((wParam & MK_CONTROL) && (wParam & MK_SHIFT))
+			{
+				::SendMessage(_hSelf, TCM_SETCURFOCUS, (isForward ? lastTabIndex : 0), 0);
+			}
+			else if (wParam & (MK_CONTROL | MK_SHIFT))
+			{
+				LRESULT tabIndex = ::SendMessage(_hSelf, TCM_GETCURSEL, 0, 0) + (isForward ? 1 : -1);
+				if (tabIndex < 0)
+				{
+					if (wParam & MK_CONTROL)
+						tabIndex = lastTabIndex; // wrap scrolling
+					else
+						return TRUE;
+				}
+				else if (tabIndex > lastTabIndex)
+				{
+					if (wParam & MK_CONTROL)
+						tabIndex = 0; // wrap scrolling
+					else
+						return TRUE;
+				}
+				::SendMessage(_hSelf, TCM_SETCURFOCUS, tabIndex, 0);
+			}
+			else if (not _isMultiLine) // don't scroll if in multi-line mode
+			{
+				RECT rcTabCtrl, rcLastTab;
+				::SendMessage(_hSelf, TCM_GETITEMRECT, lastTabIndex, (LPARAM)&rcLastTab);
+				::GetClientRect(_hSelf, &rcTabCtrl);
+
+				// get index of the first visible tab
+				TC_HITTESTINFO hti;
+				LONG xy = NppParameters::getInstance()->_dpiManager.scaleX(12); // an arbitrary coordinate inside the first visible tab
+				hti.pt = { xy, xy };
+				LRESULT scrollTabIndex = ::SendMessage(_hSelf, TCM_HITTEST, 0, (LPARAM)&hti);
+
+				if (scrollTabIndex < 1 && (_isVertical ? rcLastTab.bottom < rcTabCtrl.bottom : rcLastTab.right < rcTabCtrl.right)) // nothing to scroll
+					return TRUE;
+
+				// maximal width/height of the msctls_updown32 class (arrow box in the tab bar), 
+				// this area may hide parts of the last tab and needs to be excluded
+				LONG maxLengthUpDownCtrl = NppParameters::getInstance()->_dpiManager.scaleX(44); // sufficient static value
+
+				// scroll forward as long as the last tab is hidden; scroll backward till the first tab
+				if ((_isVertical ? ((rcTabCtrl.bottom - rcLastTab.bottom) < maxLengthUpDownCtrl) : ((rcTabCtrl.right - rcLastTab.right) < maxLengthUpDownCtrl)) || not isForward)
+				{
+					if (isForward)
+						++scrollTabIndex;
+					else
+						--scrollTabIndex;
+
+					if (scrollTabIndex < 0 || scrollTabIndex > lastTabIndex)
+						return TRUE;
+
+					// clear hover state of the close button,
+					// WM_MOUSEMOVE won't handle this properly since the tab position will change
+					if (_isCloseHover)
+					{
+						_isCloseHover = false;
+						::InvalidateRect(_hSelf, &_currentHoverTabRect, false);
+					}
+
+					::SendMessage(_hSelf, WM_HSCROLL, MAKEWPARAM(SB_THUMBPOSITION, scrollTabIndex), 0);
+				}
+			}
+			return TRUE;
+		}
+
 		case WM_LBUTTONDOWN :
 		{
 			if (_drawTabCloseButton)