window: avoid invalidating on mouse move

We weren't didn't treat the "No existing hyperlink, No new hyperlink"
case as no change in hyperlink, and were invalidating the window on
every mouse except for those over text with a hyperlink.

docs/changelog.md

@@ -14,6 +14,7 @@ brief notes about them may accumulate here.
 * Added `--config-file` CLI option to specify an alternate config file location. [Read more about config file resolution](config/files.md). Thanks to [@bew](https://github.com/bew)! [#459](https://github.com/wez/wezterm/pull/459)
 * Fixed an issue where large pastes could result in a hang
 * Updated bundled JetBrainsMono font to version 2.225
+* Fixed an issue where the window would be redrawn on mouse move. This was most noticeable as a laggy mouse pointer when moving the mouse across a window running on the nouveau display driver on X11 and Wayland systems
 
 ### 20210203-095643-70a364eb
 

wezterm-gui/src/gui/termwindow.rs

@@ -3710,6 +3710,9 @@ impl TermWindow {
             (Some(old_link), Some(new_link)) if Arc::ptr_eq(&old_link, &new_link) => {
                 // Unchanged
             }
+            (None, None) => {
+                // Unchanged
+            }
             (_, rhs) => {
                 // We're hovering over a different URL, so invalidate and repaint
                 // so that we render the underline correctly