Cancel mouse selection before swapping document

To avoid the mouse handling code getting confused

Author: niik

src/edit/methods.js

@@ -423,6 +423,8 @@ export default function(CodeMirror) {
     swapDoc: methodOp(function(doc) {
       let old = this.doc
       old.cm = null
+      // Cancel the current text selection if any (#5821)
+      if (this.state.selectingText) this.state.selectingText()
       attachDoc(this, doc)
       clearCaches(this)
       this.display.input.reset()

src/edit/mouse_events.js

@@ -305,8 +305,13 @@ function leftButtonSelect(cm, event, start, behavior) {
   function done(e) {
     cm.state.selectingText = false
     counter = Infinity
-    e_preventDefault(e)
-    display.input.focus()
+    // If e is null or undefined we interpret this as someone trying
+    // to explicitly cancel the selection rather than the user
+    // letting go of the mouse button.
+    if (e) {
+      e_preventDefault(e)
+      display.input.focus()
+    }
     off(display.wrapper.ownerDocument, "mousemove", move)
     off(display.wrapper.ownerDocument, "mouseup", up)
     doc.history.lastSelOrigin = null