Fix End of Doc Bugs (#112)

### Description

Fixes a few end-of-document bugs:
- `rectForOffset` had an incorrect `!=` instead of a `<` check for the end of the document.
- `mouseDown` needed to be looser on finding content runs for the end of documents.

Adds test cases that would have caught both these bugs.

### Related Issues

N/A

### Checklist

<!--- Add things that are not yet implemented above -->

- [x] I read and understood the [contributing guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md) as well as the [code of conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md)
- [x] The issues this PR addresses are related to each other
- [x] My changes generate no new warnings
- [x] My code builds and runs on my machine
- [x] My changes are all related to the related issue above
- [x] I documented my code

### Screenshots

N/A

Author: thecoolwinter

Sources/CodeEditTextView/TextLayoutManager/TextLayoutManager+Public.swift

@@ -166,7 +166,7 @@ extension TextLayoutManager {
     /// - Parameter offset: The offset to create the rect for.
     /// - Returns: The found rect for the given offset.
     public func rectForOffset(_ offset: Int) -> CGRect? {
-        guard offset != lineStorage.length else {
+        guard offset < lineStorage.length else {
             return rectForEndOffset()
         }
         guard let linePosition = determineVisiblePosition(for: lineStorage.getLine(atOffset: offset))?.position else {

Sources/CodeEditTextView/TextView/TextView+Mouse.swift

@@ -12,13 +12,13 @@ extension TextView {
         // Set cursor
         guard isSelectable,
               event.type == .leftMouseDown,
-              let offset = layoutManager.textOffsetAtPoint(self.convert(event.locationInWindow, from: nil)),
-              let content = layoutManager.contentRun(at: offset) else {
+              let offset = layoutManager.textOffsetAtPoint(self.convert(event.locationInWindow, from: nil)) else {
             super.mouseDown(with: event)
             return
         }
 
-        if case let .attachment(attachment) = content.data, event.clickCount < 3 {
+        if let content = layoutManager.contentRun(at: offset),
+           case let .attachment(attachment) = content.data, event.clickCount < 3 {
             handleAttachmentClick(event: event, offset: offset, attachment: attachment)
             return
         }

Tests/CodeEditTextViewTests/LayoutManager/TextLayoutManagerTests.swift

@@ -228,4 +228,26 @@ struct TextLayoutManagerTests {
         let invalidatedLineIds = layoutManager.layoutLines()
         #expect(Set(expectedLineIds) == invalidatedLineIds)
     }
+
+    @Test
+    func rectForOffsetReturnsValueAfterEndOfDoc() throws {
+        layoutManager.layoutLines(in: NSRect(x: 0, y: 0, width: 1000, height: 1000))
+
+        for idx in 0..<10 {
+            // This should return something even after the end of the document.
+            #expect(layoutManager.rectForOffset(idx) != nil, "Failed to find rect for offset: \(idx)")
+        }
+    }
+
+    @Test
+    func textOffsetForPointReturnsValuesEverywhere() throws {
+        layoutManager.layoutLines(in: NSRect(x: 0, y: 0, width: 1000, height: 1000))
+
+        // textOffsetAtPoint is valid *everywhere*. It should always return something.
+        for xPos in 0..<1000 {
+            for yPos in 0..<1000 {
+                #expect(layoutManager.textOffsetAtPoint(CGPoint(x: xPos, y: yPos)) != nil)
+            }
+        }
+    }
 }

Tests/CodeEditTextViewTests/TextSelectionManagerTests.swift

@@ -217,4 +217,10 @@ final class TextSelectionManagerTests: XCTestCase {
             )
         }
     }
+
+    func test_selectionEndOfDocumentHasXPos() {
+        let selectionManager = selectionManager("1\n2\n3\n")
+        selectionManager.setSelectedRange(NSRange(location: 6, length: 0)) // Beyond text.length, end of doc
+        XCTAssertNotNil(selectionManager.textSelections.first?.suggestedXPos)
+    }
 }