shift-click extends selection

TODO.md

@@ -18,7 +18,6 @@ Correctness:
 Improvements:
 
 * scrollback: configurable
-* shift-click and drag to continue selection
 
 Mac:
 

src/Surface.zig

@@ -1514,6 +1514,17 @@ pub fn mouseButtonCallback(
     self.mouse.click_state[@intCast(usize, @enumToInt(button))] = action;
     self.mouse.mods = @bitCast(input.Mods, mods);
 
+    // Shift-click continues the previous mouse state. cursorPosCallback
+    // will also do a mouse report so we don't need to do any the logic
+    // below.
+    if (button == .left and action == .press) {
+        if (mods.shift and self.mouse.left_click_count > 0) {
+            const pos = try self.rt_surface.getCursorPos();
+            try self.cursorPosCallback(pos);
+            return;
+        }
+    }
+
     self.renderer_state.mutex.lock();
     defer self.renderer_state.mutex.unlock();