Bugfix: only round to the nearest cell for selection

[carlos-zamora]

Summary of the Pull Request

Fixes a bug where VT mouse mode would round to the nearest cell when clicking the mouse button.
The fix is to round to the nearest cell only when we're selecting text. The other scenarios affected are:

• clicking on a hyperlink
• vt mouse mode
• where the context menu is anchored

Really the most notable ones were the first two. So now, we use the position of the cell we clicked on. We only round for selection.

References and Relevant Issues

Follow-up to #18106

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Opened Midnight Commander in Ubuntu and clicked between the two panes.

• Before: threshold was too early to switch between panes
• After: threshold is clearly separated between the outline of the two panes

[DHowett]

Do away with the GH# references - in a way, it's like saying "this is related to the selection changes". Well, every change here is related to selection ;)