Undo history is lost when highlighting text (with mouse) in insert mode

Describe the bug

Did a sequence of operations that deleted the text without storing it in history.

To Reproduce

It reproduces every time. I did something like this:

• Entered the text in insert mode
• Highlighted text with mouse, without exiting insert mode
• After highlighting the text, I was still in insert mode, so I typed something - e.g. "x" - and it replaced my highlighted text.
• When I tried to undo, I went to state from before my first bullet point.

Consequences:

• If I highlight the text I was typing, it's deleted permanently with no way to recover.
• It's not something user usually wants, but it happened by accident - I typed "x" because I wanted to delete the highlighted text, not noticing I'm still in insert mode.
• VS Code undo history is identical to Vim history, so I can't fallback to VS Code controls.

Expected behavior
I want the undo history to be 100% reliable - to be saved every time I'm deleting more than one character at a time, e.g. deleting highlighted text. This is my expectation, because it's true for text editors I know, including native Vim and native VS Code.

Environment (please complete the following information):

• Extension (VsCodeVim) version: 1.14.2
• VSCode version: 1.45.0
• OS: Ubuntu 19.10

No source file or test is named. First reproduce the sequence in VSCode with VSCodeVim 1.14.2: type in insert mode, highlight text with the mouse, replace it, and undo; then locate the insert-mode mouse-selection and undo-history entry points. Done means the replacement of highlighted text can be undone without losing the preceding insert history.