Problem

In src/vs/editor/browser/controller/mouseTarget.ts:1136-1142, the shadowCaretRangeFromPoint function calls getComputedStyle(el, null) six separate times to retrieve individual CSS font properties (font-style, font-variant, font-weight, font-size, line-height, font-family) and then concatenates them into a font shorthand string.

Each getComputedStyle() call forces a synchronous style recalculation, which is expensive. Since this function runs on every mouse move over the editor viewport's shadow DOM content, the overhead is multiplied across every mousemove event in the editor.

Fix

Cache the getComputedStyle() result once in a local variable, then call .getPropertyValue() on the cached result:

- const fontStyle = elWindow.getComputedStyle(el, null).getPropertyValue('font-style');
- const fontVariant = elWindow.getComputedStyle(el, null).getPropertyValue('font-variant');
- const fontWeight = elWindow.getComputedStyle(el, null).getPropertyValue('font-weight');
- const fontSize = elWindow.getComputedStyle(el, null).getPropertyValue('font-size');
- const lineHeight = elWindow.getComputedStyle(el, null).getPropertyValue('line-height');
- const fontFamily = elWindow.getComputedStyle(el, null).getPropertyValue('font-family');
+ const computedStyle = elWindow.getComputedStyle(el, null);
+ const fontStyle = computedStyle.getPropertyValue('font-style');
+ const fontVariant = computedStyle.getPropertyValue('font-variant');
+ const fontWeight = computedStyle.getPropertyValue('font-weight');
+ const fontSize = computedStyle.getPropertyValue('font-size');
+ const lineHeight = computedStyle.getPropertyValue('line-height');
+ const fontFamily = computedStyle.getPropertyValue('font-family');

Impact

• 6x reduction in forced style recalculations on every mouse move over editor content
• No functional change — identical output, same behavior
• Low-risk, one-line addition, single file change

File

src/vs/editor/browser/controller/mouseTarget.ts — shadowCaretRangeFromPoint function

Related

This is a well-known browser performance best practice — MDN recommends caching getComputedStyle() results when reading multiple properties: https://developer.mozilla.org/en-US/docs/Web/API/Window/getComputedStyle#notes