🐛 Fix atom/tabs#461

On Linux, when the user performs a middle-button mouse click, Chromium
fires both a mouse-down event *and* a paste event. This commit teaches
the TextEditorComponent to ignore the paste event.

When the user performs a middle-mouse click on a tab, we get close the
tab and attempt to prevent Chromium's default processing for the event.
[1] This prevents Chromium's default processing for the *mouse down*
event, but then Chromium also fires a *paste* event, and that event
pastes the clipboard's current content into the newly-focused text
editor. 🙀

Since Atom already has its own logic for handling pasting, we
shouldn't (🤞) need to handle browser paste events. By ignoring the
browser paste events on Linux, we fix atom/tabs#461.

[1]
ce1d92e0ab/lib/tab-bar-view.coffee (L416-L418)

spec/text-editor-component-spec.js

@@ -2687,18 +2687,42 @@ describe('TextEditorComponent', () => {
         expect(component.getScrollLeft()).toBe(maxScrollLeft)
       })
 
-      it('positions the cursor on clicking the middle mouse button on Linux', async () => {
-        // The browser synthesizes the paste as a textInput event on mouseup
-        // so it is not possible to test it here.
+      it('pastes the previously selected text when clicking the middle mouse button on Linux', async () => {
+        spyOn(electron.ipcRenderer, 'send').andCallFake(function (eventName, selectedText) {
+          if (eventName === 'write-text-to-selection-clipboard') {
+            clipboard.writeText(selectedText, 'selection')
+          }
+        })
+
         const {component, editor} = buildComponent({platform: 'linux'})
 
+        // Middle mouse pasting.
         editor.setSelectedBufferRange([[1, 6], [1, 10]])
+        await conditionPromise(() => TextEditor.clipboard.read() === 'sort')
         component.didMouseDownOnContent({
           button: 1,
           clientX: clientLeftForCharacter(component, 10, 0),
           clientY: clientTopForLine(component, 10)
         })
-        expect(editor.getSelectedBufferRange()).toEqual([[10, 0], [10, 0]])
+        expect(TextEditor.clipboard.read()).toBe('sort')
+        expect(editor.lineTextForBufferRow(10)).toBe('sort')
+        editor.undo()
+
+        // Ensure left clicks don't interfere.
+        editor.setSelectedBufferRange([[1, 2], [1, 5]])
+        await conditionPromise(() => TextEditor.clipboard.read() === 'var')
+        component.didMouseDownOnContent({
+          button: 0,
+          detail: 1,
+          clientX: clientLeftForCharacter(component, 10, 0),
+          clientY: clientTopForLine(component, 10)
+        })
+        component.didMouseDownOnContent({
+          button: 1,
+          clientX: clientLeftForCharacter(component, 10, 0),
+          clientY: clientTopForLine(component, 10)
+        })
+        expect(editor.lineTextForBufferRow(10)).toBe('var')
       })
     })